P bond exists in molecules. Chemical bond. Formation of sigma bond and pi bond

The nature of the chemical bond. Quantum-mechanical interpretation of the mechanism of chemical bond formation.

Types of bonds: covalent, ionic, coordination (donor-acceptor), metallic, hydrogen.

Bond characteristics: bond energy and length, directivity, saturation, electric dipole moments, effective charges of atoms, degree of ionicity.

The method of valence bonds (VS). Sigma and pi bonds. Types of hybridization of atomic orbitals and geometry of molecules. unshared electron pairs of molecules.

The method of molecular orbitals (MO) and features of the wave function used in it. Bonding and loosening molecular orbitals. The principles of filling them with electrons, the order and energy of bonds. Bonds in diatomic homonuclear molecules.

Properties of chemical bonds in the solid state of matter. Properties of ionic crystals. Metal bond and structure of metal crystals. Specific properties of metals. Molecular crystals and their properties.

Application of the theory of chemical bonding in chemistry and biology. Energy of covalent bonds and energetics of chemical reactions. Prediction of the geometry of molecules. Flexibility of biomolecules as a result of free rotation around s-bonds. The interaction of biomolecules with water as a result of the formation of hydrogen bonds and the interaction of water dipoles with atoms that have significant charges.

Option 1

1. What bond is called ionic? Show the mechanism of the formation of an ionic bond using the example of the formation of potassium fluoride. Is it possible to speak about the CI molecule for the solid state of matter?

2. Which of the following molecules has a p-bond? CH4; N 2 ; BeCl2; CO2. Support your answer with graphic formulas.

3. What is the mechanism of variable valence of elements? Why does sulfur exhibit a variable valence, oxygen is always no more than divalent?

4. Designate the type of hybridization of orbitals in CH 4 , MgCl 2 , BF 3 molecules.

Option 2

1. What is the peculiarity of a typical covalent bond? Show the mechanism for the emergence of this connection in a generalized schematic form.

2. From among the compounds listed below, write out in two columns the molecules with single and multiple bonds. Those in which there is a π-bond, underline.

C 2 H 4, NH 3, N 2, CCl 4, SO 2, H 2 O.

3. How does the nature of the chemical bond of atoms affect the properties of substances (the ability to dissociate, t, etc.)?

4. Draw a picture of the Sp 2 hybridization process. Give an example of the corresponding molecule and indicate its geometry.

Option 3

1. How does the energy reserve of molecules change in comparison with the energy reserve of disparate atoms? Which molecule is stronger: H 2 (E CB = 431.8 kJ) or N 2 (E CB = 945 kJ)?

2. What determines the value of the covalence of an element? Give the graphic formulas of the molecules N 2, NH 3, NO and determine the covalence of nitrogen in each of them.

3. What is called the hybridization of orbitals? Draw one hybrid orbital and explain why hybrid bonds form a stronger bond than non-hybrid ones.

4. Give a general description of crystalline substances and name the types of crystal lattices.

Option 4

1. List the main types of chemical bonds and give one example of chemical compounds corresponding to these types of bonds.

2. Draw two possible ways of overlapping p-electron clouds.

3. What is called the length of the dipole and the dipole moment of the molecule? What determines the magnitude of the dipole moment?

4. From the molecules listed below, write down those in which there are Sp-hybrid orbitals, and indicate their geometry.

BeCl 2 , BCl 3 , H 2 O, C 2 H 2 .

Option 5

1. What is the peculiarity of the donor-acceptor bond? Show its mechanism in a generalized schematic form and with an example.

2. What determines the value of the covalence of an atom in a molecule? Does covalence have a sign? Determine the covalence of sulfur in the H 2 S molecule and the ion using their graphical formulas.

3. How many σ- and π-bonds are in the N+ molecule, ion?

4. Why does the CaCl 2 molecule (in vapor) have a linear shape, the BCl 3 molecule is triangular - flat, and the CCl 4 molecule - tetrahedral?

Option 6

1. What is the physical nature of a typical covalent bond in accordance with the concepts of wave mechanics? What should be the spins of the electrons of the interacting atoms so that they can enter into a chemical interaction with each other?

2. How does the modern theory of chemical bonding explain the variable valence of elements? Give an example.

3. Explain using graphical formulas? why, in the presence of polar bonds in CO 2 and SO 2 molecules, one of them is non-polar, and the other is polar.

4. Write down the chemical compounds in the formation of which Sp 2 hybrid orbitals C 2 H 4 participate; CH4; BCl 3 ; C2H2.

Option 7

1. In what cases and how does a hydrogen bond occur? Give examples.

2. Write out those of the molecules below, in which there is a typical covalent bond between PCl 3 atoms; N 2 ; K2S; SO3. Give their graphic formulas.

3. What principles and rules govern the filling of both atomic and molecular orbitals? How is the number of chemical bonds in a molecule determined by the MO method?

4. Which of the following molecules have an angular shape? CO 2 , SO 2 , H 2 O.

Option 8

1. What are the features of a metallic bond?

2. How many idle electrons do Al and Se atoms have in the ground state? What process determines the possibility of increasing the covalence of these elements to a value corresponding to the number of their group in the system of D. I. Mendeleev?

3. In which of the given molecules do the absolute values, oxidation states, and covalences of the underlined elements not match?

N 2 , H 2 , NH 3 , C 2 H 2 .

Justify your answer with graphic formulas.

4. Sketch the process of Sp 3 -hybridization of orbitals. Give an example of a molecule in which this type of hybridization occurs.

Option 9

1. For which of the following molecules are intermolecular hydrogen bonds possible and why? CaH 2, H 2 O, HF 2, CH 4.

2. What determines the degree of polarization of the bond between atoms in a molecule and what is its quantitative characteristic?

3. How many σ- and π-bonds are in a CO 2 molecule? What type of hybridization of carbon atom orbitals is here?

4. Which of the listed substances have molecular and which ionic crystal lattices in the solid state?

NaJ, H 2 O, K 2 SO 4 , CO 2 , J 2 .

Option 10

1. Draw the structure of H 2, N 2 and NH 3 molecules using the valence schemes (VS) method. What is the type of bond between the atoms of these molecules? Which of the molecules has π bonds?

2. By the type of chemical bond, determine which of the following substances a) has the greatest ability to dissociate; b) the lowest melting point; c) the highest boiling point. HF; Cl2.

3. What is the direction of the covalent bond? Using the example of the structure of a water molecule, show how the direction of the bond affects the geometry of the molecule.

4. In which of the listed molecules the bond angles between atoms are 180°?. What type of orbital hybridization does this explain?

CH 4 , BF 3 , MgCl 2 , C 2 H 2 .

Option 11

1. Which electrons: paired or idle - determine the possible number of typical covalent bonds of an atom in a given energy state? As an example, consider the sulfur atom.

2. How do σ- and π-bonds differ from each other? Can hybrid orbitals form a π bond? Compare the strength of π- and σ-bonds.

3. Draw a diagram of the Sp-hybridization of orbitals and write out those of the given molecules that have this type of hybridization.

BeCl 2 , CH 4 , AlF 3 , C 2 H 2 .

4. Give a general description of the features of amorphous bodies.

Option 12

1. What is the difference between covalent-non-polar and covalent-polar bonds? Explain with examples when they occur.

2. Specify the types of bonds in the following compounds and ions:

CsF, 2+ , Cl 2 , SO 3 .

3. How many hybrid orbitals are formed during Sp 3 hybridization? What is the geometry of the CH 4 molecule in which this type of hybridization occurs?

4. What types of intermolecular interactions are known?

Option 13

1. According to the electronegativity values ​​of sulfur, chlorine and sodium atoms, determine which of them form an ionic bond with each other and which form a covalent bond.

2. Redraw the table and complete it for the underlined atoms.

3. Why can phosphorus form compounds PCl 3 and PCl 5 , and nitrogen - only NCl 3 ? To which atom is the electron pair shifted in all these molecules?

4. Which of the following molecules have the shape of a tetrahedron and why?

Option 14

1. What determines the value of the electrovalency of an element in ionic compounds? Designate the electrovalence in the compounds K 2 S, MgCl 2, AlCl 3. Does it match the oxidation state?

2. What is the difference between the method of molecular orbitals (MO) and the method of valence bonds (BC)? Give the schemes for the formation of a hydrogen molecule by the VS method and the MO method.

3. What types of bonds are there in the NH 4 Cl molecule? Show them on the electronic diagram of the structure of the molecule.

4. Indicate the types of hybridization of orbitals and the geometry of BeF 2 , СH 4 , BCl 3 molecules.

covalent bond. The structure of the water molecule

Task 61.
What chemical bond is called a covalent bond? How can one explain the direction of a covalent bond? How does the method of valence bonds (BC) explain the structure of the water molecule?
Decision:
Communication carried out due to the formation of electron pairs, equally belonging to both atoms is called covalent non-polar. Covalent bonds are oriented in space in a certain way, that is, they have a direction. The reason that molecules can have a linear planar or some other structure is that atoms use different orbitals and different numbers of them to form bonds. Molecules that have a dipole moment are not linear, while molecules that do not have a dipole moment are linear.

The water molecule H 2 O has a dipole moment, which means that it has a nonlinear structure. One oxygen atom and two hydrogen atoms participate in the formation of bonds between oxygen and hydrogen atoms. Oxygen is the neutral atom in the water molecule, and it has four electron pairs, two lone pairs and two shared ones, which are formed by one s-electron and one p-electron of oxygen. Such a molecule has a tetrahedral structure in the center of the tetrahedron there is an oxygen atom, and at the corners of the tetrahedron there are two hydrogen atoms and two lone electron pairs of oxygen. In such a molecule, the angle between bonds should be equal to 109.5 0 . If the water molecule were flat, then the angle HOH should be 90 0 . But X-ray diffraction analysis of water molecules shows that the HOH angle is 104.5 0 . This explains that the water molecule does not have a linear shape, but has the shape of a distorted tetrahedron. This is explained by the fact that the oxygen atom undergoes sp 3 hybridization, when one s-orbital and three p-orbitals of the oxygen atom hybridize, forming four equivalent sp 3 hybrid orbitals. Of the four sp 3 hybrid orbitals, two are occupied by the s orbitals of the hydrogen atom. The difference between the values ​​of the bond angle and the tetrahedral angle is explained by the fact that the repulsion between lone electron pairs is greater than between bonding ones.

Polar covalent bond

Task 62.
What covalent bond is called polar? What is the quantitative measure of the polarity of a covalent bond? Based on the electronegativity values ​​of the atoms of the corresponding elements, determine which of the bonds: HCl, ICl, BrF is the most polar.
Decision:
A covalent bond formed by different atoms is called a polar bond. For example, H - Cl; the center of gravity of a negative charge (associated with electrons) does not coincide with the center of gravity of a positive charge (associated with the charge of the atomic nucleus). The electron density of common electrons is shifted to one of the atoms, which has a higher electronegativity value, to a greater extent. In H:Cl, the shared electron pair is biased towards the most electronegative chlorine atom. The polarity of the bond is quantified by the dipole moment (), which is the product of the dipole length (l) - the distance between two equal and opposite charges +g and -g by the absolute value of the charge: = lg. The dipole moments HCI, HBr, HI are equal to 1.04, respectively; 0.79; 0.38 D. Dipole moments of molecules are usually measured in debyes (D)*: 1D = 3.33 . 10 -30 C . m.

The dipole moment is a vector quantity and is directed along the dipole axis from a negative charge to a positive one. The bond dipole moment provides valuable information about the behavior of the molecule as a whole. Along with the dipole moment, a characteristic called the electronegativity of the element (EO) is used to assess the degree of polarity of the bond. EO is the ability of an atom to attract the valence electrons of other atoms to itself. The values ​​of EO elements are given in special scales (tables).

The EO values ​​of hydrogen, chlorine, bromine, iodine, fluorine, respectively, are: 2.1; 3.0; 2.8; 2.5; 4.0. Based on the values ​​of the EO elements in the compounds

the most polar bond in the BrF molecule, since the difference in electronegativity between fluorine and bromine is the largest - 1.2 (4.0 - 2.8 = 1.2) than that of HCl and ICl.

Donor-acceptor bond

Task 63.
What method of covalent bond formation is called donor-acceptor? What chemical bonds are present in NH 4+ and BF 4- ions? Specify the donor and acceptor.
Decision:

A donor-acceptor bond is a covalent bond in which only one of the atoms participating in the bond provides a shared pair of electrons. In this case, one of the atoms is a donor - a supplier of an electron pair, and the other is an acceptor - a supplier of a free quantum orbital.

The ammonium cation NH 4+ is formed by the donor-acceptor mechanism:

It has the shape of a regular tetrahedron:

In the ammonium ion, each hydrogen atom is bonded to the nitrogen atom by a common electron pair, one of which is realized by the donor-acceptor mechanism. It is important to note that the H - N bonds formed by various mechanisms do not have any differences, i.e. they are all equivalent. The donor is a nitrogen atom, and the acceptor is a hydrogen atom.

The BF 4- ion is formed from BF 3 and the F- ion. This ion is formed due to the fact that the unshared electron pair of the F- ion is “embedded” in the valence shell of the boron atom of the covalently bound BF 3 molecule:

In the BF 4 ion, the fluorine ion is the donor, and the boron atom of the BF 3 molecule is the acceptor.

The donor-acceptor bond in the structural formulas is depicted by an arrow that is directed from the donor to the acceptor.

Valence bond method (BC)

Task 64.
How does the method of valence bonds (BC) explain the linear structure of the BeCl 2 molecule and the tetrahedral CH 4?
Decision

a) Representations of the method of valence bonds make it possible to explain the geometry of many molecules. Thus, the BeCl2 molecule consists of one beryllium atom and two chlorine atoms. An excited beryllium atom has one s-electron and one p-electron. When BeCl 2 is formed, two covalent bonds appear. One of them should be an s - p bond formed due to the overlap of the s-cloud of the beryllium atom and the p-cloud of the chlorine atom, the other (p - p bond) due to the overlap of the p-cloud of the beryllium atom and the p-cloud of the chlorine atom.

p - p bond and s - p can be located at an angle relative to each other, i.e. the BeCl 2 molecule must be angular, but it is precisely established that the BeCl 2 molecule has a linear structure, and both bonds are equal in energy and length. The concept of hybridization of atomic orbitals is used to explain the geometry of the BeCl 2 molecule. The essence of the concept of atomic orbitals is that atomic orbitals can be geometrically modified and mixed with each other in such a way as to provide the greatest overlap with the orbits of other atoms and, therefore, the greatest gain in energy. This is achieved if, instead of orbitals having different shapes and energies, hybrid orbitals of the same shape and energy appear, which are linear combinations of the original atomic orbitals. So in the Be atom, the s-orbital and p-orbital interact, their energies are aligned and two identical sp-hybrid orbitals are formed. The two generated sp-hybrid electron clouds have the same energy and an asymmetric shape, which provides more overlap with p-electron clouds of the chlorine atom than overlap with pure unhybridized s- and p-clouds. Two hybrid sp-clouds are located relative to each other and the atomic nucleus at an angle of 180 0:

Rice. 1. Triatomic molecule BeCl 2

As a result of this arrangement of hybrid clouds, the BeCl 2 molecule has a linear structure.

b) The CH 4 molecule consists of one carbon atom and four hydrogen atoms, between which there are four covalent bonds. An excited carbon atom has four unpaired electrons, one in the s orbital and three in the p orbitals:

Filling the external energy level of the carbon atom in the ground state:

Filling the external energy level of the carbon atom in an excited state:

Of the four bonds in the CH 4 molecule, there should be one s - s and three s - p bonds formed due to the overlapping of the orbitals of the carbon atom with the s-orbital of hydrogen atoms. As a result of this overlap, an s - s bond should be formed, different from the three s - p bonds of length and energy, and located at an angle of about 125 0 to any of them. However, it is precisely established that the CH 4 molecule has the shape of a tetrahedron with an angle between bonds of 109.5 0, and all bonds are equivalent in length and energy. The tetrahedral structure of the CH 4 molecule can be explained by sp 3 hybridization. The carbon atom contains four sp 3 hybrid orbitals, resulting from a linear combination of an s orbital and three p orbitals. Four sp3-hybrid orbitals are located relative to each other at an angle of 109.5 0 . They are directed to the vertices of the tetrahedron, in the center of which is the nucleus of the carbon atom (Fig. 2.).

Rice. 2. Scheme of the structure of the CH 4 molecule;
Methane, there are no non-bonding electron pairs.

Thus, four equivalent chemical bonds are formed in the CH4 molecule due to the overlapping of sp3-hybrid orbitals of the carbon atom with s-orbitals of carbon atoms.

Formation of sigma bond and pi bond

Task 65.
Which covalent bond is called a -bond and which -bond? Explain the structure of the nitrogen molecule as an example.
Decision:
A bond formed by overlapping along a line connecting two atoms is called -bond (any simple bond) or “If the overlap of atomic orbitals occurs on the internuclear axis, then a sigma bond is formed (-connection). A sigma bond is formed by overlapping two s-orbitals (s-s bond), one s- and one p-orbital (s-p bond), two p-orbitals (p-p bond), one s- and one d -orbital (s - d bond), one p- and one d-orbital (p - d bond).

Options for overlapping atomic orbitals leading to the formation

162774 0

Each atom has a certain number of electrons.

Entering into chemical reactions, atoms donate, acquire, or socialize electrons, reaching the most stable electronic configuration. The configuration with the lowest energy is the most stable (as in noble gas atoms). This pattern is called the "octet rule" (Fig. 1).

Rice. one.

This rule applies to all connection types. Electronic bonds between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that eventually form living systems. They differ from crystals in their continuous metabolism. However, many chemical reactions proceed according to the mechanisms electronic transfer, which play an important role in the energy processes in the body.

A chemical bond is a force that holds together two or more atoms, ions, molecules, or any combination of them..

The nature of the chemical bond is universal: it is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons in the outer shell of atoms. The ability of an atom to form chemical bonds is called valence, or oxidation state. Valence is related to the concept of valence electrons- electrons that form chemical bonds, that is, those located in the most high-energy orbitals. Accordingly, the outer shell of an atom containing these orbitals is called valence shell. At present, it is not enough to indicate the presence of a chemical bond, but it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.

The first type of connection isionic connection

According to Lewis and Kossel's electronic theory of valency, atoms can achieve a stable electronic configuration in two ways: first, by losing electrons, becoming cations, secondly, acquiring them, turning into anions. As a result of electron transfer, due to the electrostatic force of attraction between ions with charges of the opposite sign, a chemical bond is formed, called Kossel " electrovalent(now called ionic).

In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from cations of T and II groups of the periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens and halogens). The bonds in ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. On fig. 2 and 3 show examples of ionic bonds corresponding to the Kossel electron transfer model.

Rice. 2.

Rice. 3. Ionic bond in the sodium chloride (NaCl) molecule

Here it is appropriate to recall some of the properties that explain the behavior of substances in nature, in particular, to consider the concept of acids and grounds.

Aqueous solutions of all these substances are electrolytes. They change color in different ways. indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that the indicators are weak acids or bases, the color of which in the undissociated and dissociated states is different.

Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds that do not contain -OH groups are insoluble, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.

Aqueous solutions of acids enter into characteristic reactions:

a) with metal oxides - with the formation of salt and water;

b) with metals - with the formation of salt and hydrogen;

c) with carbonates - with the formation of salt, CO 2 and H 2 O.

The properties of acids and bases are described by several theories. In accordance with the theory of S.A. Arrhenius, an acid is a substance that dissociates to form ions H+ , while the base forms ions IS HE- . This theory does not take into account the existence of organic bases that do not have hydroxyl groups.

In line with proton Bronsted and Lowry's theory, an acid is a substance containing molecules or ions that donate protons ( donors protons), and the base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in a hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also carried out in the absence of a solvent or with a non-aqueous solvent.

For example, in the reaction between ammonia NH 3 (weak base) and hydrogen chloride in the gas phase, solid ammonium chloride is formed, and in an equilibrium mixture of two substances there are always 4 particles, two of which are acids, and the other two are bases:

This equilibrium mixture consists of two conjugated pairs of acids and bases:

1)NH 4+ and NH 3

2) HCl and Cl ‑

Here, in each conjugated pair, the acid and base differ by one proton. Every acid has a conjugate base. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.

The Bronsted-Lowry theory makes it possible to explain the unique role of water for the life of the biosphere. Water, depending on the substance interacting with it, can exhibit the properties of either an acid or a base. For example, in reactions with aqueous solutions of acetic acid, water is a base, and with aqueous solutions of ammonia, it is an acid.

1) CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 SOO- . Here the acetic acid molecule donates a proton to the water molecule;

2) NH3 + H 2 O ↔ NH4 + + IS HE- . Here the ammonia molecule accepts a proton from the water molecule.

Thus, water can form two conjugated pairs:

1) H 2 O(acid) and IS HE- (conjugate base)

2) H 3 O+ (acid) and H 2 O(conjugate base).

In the first case, water donates a proton, and in the second, it accepts it.

Such a property is called amphiprotonity. Substances that can react as both acids and bases are called amphoteric. In nature, such substances are often found. For example, amino acids can form salts with both acids and bases. Therefore, peptides readily form coordination compounds with the metal ions present.

Thus, the characteristic property of an ionic bond is the complete displacement of a bunch of binding electrons to one of the nuclei. This means that there is a region between the ions where the electron density is almost zero.

The second type of connection iscovalent connection

Atoms can form stable electronic configurations by sharing electrons.

Such a bond is formed when a pair of electrons is shared one at a time. from each atom. In this case, the socialized bond electrons are distributed equally among the atoms. An example of a covalent bond is homonuclear diatomic H molecules 2 , N 2 , F 2. Allotropes have the same type of bond. O 2 and ozone O 3 and for a polyatomic molecule S 8 and also heteronuclear molecules hydrogen chloride Hcl, carbon dioxide CO 2, methane CH 4, ethanol With 2 H 5 IS HE, sulfur hexafluoride SF 6, acetylene With 2 H 2. All these molecules have the same common electrons, and their bonds are saturated and directed in the same way (Fig. 4).

For biologists, it is important that the covalent radii of atoms in double and triple bonds are reduced compared to a single bond.

Rice. 4. Covalent bond in the Cl 2 molecule.

Ionic and covalent types of bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.

Compounds of two elements located at opposite ends of the same or different periods of the Mendeleev system predominantly form ionic bonds. As the elements approach within the period, the ionic nature of their compounds decreases, and the covalent character increases. For example, the halides and oxides of the elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4 , CaCO 3 , KNO 3 , CaO, NaOH), and the same compounds of the elements on the right side of the table are covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C6H5OH, glucose C 6 H 12 O 6, ethanol C 2 H 5 OH).

The covalent bond, in turn, has another modification.

In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It is called donor electron pair. An atom that socializes this pair of electrons with a donor is called acceptor electron pair. This type of covalent bond is called coordination (donor-acceptor, ordative) communication(Fig. 5). This type of bond is most important for biology and medicine, since the chemistry of the most important d-elements for metabolism is largely described by coordination bonds.

Pic. 5.

As a rule, in a complex compound, a metal atom acts as an electron pair acceptor; on the contrary, in ionic and covalent bonds, the metal atom is an electron donor.

The essence of the covalent bond and its variety - the coordination bond - can be clarified with the help of another theory of acids and bases, proposed by GN. Lewis. He somewhat expanded the semantic concept of the terms "acid" and "base" according to the Bronsted-Lowry theory. The Lewis theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.

According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone pair of electrons, which, by donating electrons, forms a covalent bond with Lewis acid.

That is, the Lewis theory expands the range of acid-base reactions also to reactions in which protons do not participate at all. Moreover, the proton itself, according to this theory, is also an acid, since it is able to accept an electron pair.

Therefore, according to this theory, cations are Lewis acids and anions are Lewis bases. The following reactions are examples:

It was noted above that the subdivision of substances into ionic and covalent ones is relative, since there is no complete transfer of an electron from metal atoms to acceptor atoms in covalent molecules. In compounds with an ionic bond, each ion is in the electric field of ions of the opposite sign, so they are mutually polarized, and their shells are deformed.

Polarizability determined by the electronic structure, charge and size of the ion; it is higher for anions than for cations. The highest polarizability among cations is for cations of larger charge and smaller size, for example, for Hg 2+ , Cd 2+ , Pb 2+ , Al 3+ , Tl 3+. Has a strong polarizing effect H+ . Since the effect of ion polarization is two-sided, it significantly changes the properties of the compounds they form.

The third type of connection -dipole-dipole connection

In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also known as van der Waals .

The strength of these interactions depends on the nature of the molecules.

There are three types of interactions: permanent dipole - permanent dipole ( dipole-dipole attraction); permanent dipole - induced dipole ( induction attraction); instantaneous dipole - induced dipole ( dispersion attraction, or London forces; rice. 6).

Rice. 6.

Only molecules with polar covalent bonds have a dipole-dipole moment ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the bond strength is 1-2 debye(1D \u003d 3.338 × 10 -30 coulomb meters - C × m).

In biochemistry, another type of bond is distinguished - hydrogen connection, which is a limiting case dipole-dipole attraction. This bond is formed by the attraction between a hydrogen atom and a small electronegative atom, most often oxygen, fluorine and nitrogen. With large atoms that have a similar electronegativity (for example, with chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one essential feature: when the binding electrons are pulled away, its nucleus - the proton - is exposed and ceases to be screened by electrons.

Therefore, the atom turns into a large dipole.

A hydrogen bond, unlike a van der Waals bond, is formed not only during intermolecular interactions, but also within one molecule - intramolecular hydrogen bond. Hydrogen bonds play an important role in biochemistry, for example, for stabilizing the structure of proteins in the form of an α-helix, or for the formation of a DNA double helix (Fig. 7).

Fig.7.

Hydrogen and van der Waals bonds are much weaker than ionic, covalent, and coordination bonds. The energy of intermolecular bonds is indicated in Table. one.

Table 1. Energy of intermolecular forces

Note: The degree of intermolecular interactions reflect the enthalpy of melting and evaporation (boiling). Ionic compounds require much more energy to separate ions than to separate molecules. The melting enthalpies of ionic compounds are much higher than those of molecular compounds.

The fourth type of connection -metallic bond

Finally, there is another type of intermolecular bonds - metal: connection of positive ions of the lattice of metals with free electrons. This type of connection does not occur in biological objects.

From a brief review of the types of bonds, one detail emerges: an important parameter of an atom or ion of a metal - an electron donor, as well as an atom - an electron acceptor is its the size.

Without going into details, we note that the covalent radii of atoms, the ionic radii of metals, and the van der Waals radii of interacting molecules increase as their atomic number in the groups of the periodic system increases. In this case, the values ​​of the ion radii are the smallest, and the van der Waals radii are the largest. As a rule, when moving down the group, the radii of all elements increase, both covalent and van der Waals.

The most important for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.

Medical bioinorganics. G.K. Barashkov

The smallest particle of a substance is a molecule formed as a result of the interaction of atoms between which there are chemical bonds or a chemical bond. The doctrine of the chemical bond is the basis of theoretical chemistry. A chemical bond occurs when two (sometimes more) atoms interact. Bond formation occurs with the release of energy.

A chemical bond is an interaction that binds individual atoms into molecules, ions, crystals.

Chemical bonding is inherently one: it is of electrostatic origin. But in various chemical compounds, the chemical bond is of various types; The most important types of chemical bonds are covalent (non-polar, polar), ionic, and metallic. Varieties of these types of bonds are donor-acceptor, hydrogen, etc. A metallic bond arises between metal atoms.

A chemical bond carried out by the formation of a common, or shared, pair or several pairs of electrons is called covalent. In the formation of one common pair of electrons, each atom contributes one electron, i.e. participates "in equal shares" (Lewis, 1916). Below are schemes for the formation of chemical bonds in H2, F2, NH3, and CH4 molecules. Electrons belonging to different atoms are designated by different symbols.

As a result of the formation of chemical bonds, each of the atoms in the molecule has a stable two- and eight-electron configuration.

When a covalent bond occurs, the electron clouds of atoms overlap with the formation of a molecular electron cloud, accompanied by a gain in energy. The molecular electron cloud is located between the centers of both nuclei and has an increased electron density compared to the density of the atomic electron cloud.

The implementation of a covalent bond is possible only in the case of antiparallel spins of unpaired electrons belonging to different atoms. With parallel spins of electrons, atoms do not attract, but repel: a covalent bond does not occur. The method of describing a chemical bond, the formation of which is associated with a common electron pair, is called the method of valence bonds (MVS).

Fundamentals of the AIM

A covalent chemical bond is formed by two electrons with oppositely directed spins, and this electron pair belongs to two atoms.

The stronger the covalent bond, the more the interacting electron clouds overlap.

When writing structural formulas, the electron pairs that cause the bond are often depicted as dashes (instead of dots representing socialized electrons).

The energy characteristic of a chemical bond is important. When a chemical bond is formed, the total energy of the system (molecule) is less than the energy of its constituent parts (atoms), i.e. EAB<ЕА+ЕB.

Valency is the property of an atom of a chemical element to attach or replace a certain number of atoms of another element. From this point of view, the valency of an atom is easiest to determine by the number of hydrogen atoms that form chemical bonds with it, or by the number of hydrogen atoms that are replaced by an atom of this element.

With the development of quantum mechanical concepts of the atom, valency began to be determined by the number of unpaired electrons involved in the formation of chemical bonds. In addition to unpaired electrons, the valency of an atom also depends on the number of empty and completely filled orbitals of the valence electron layer.

The binding energy is the energy released when a molecule is formed from atoms. The binding energy is usually expressed in kJ/mol (or kcal/mol). This is one of the most important characteristics of a chemical bond. A system that contains less energy is more stable. It is known, for example, that hydrogen atoms tend to combine into a molecule. This means that a system consisting of H2 molecules contains less energy than a system consisting of the same number of H atoms, but not combined into molecules.

Rice. 2.1 Dependence of the potential energy E of a system of two hydrogen atoms on the internuclear distance r: 1 - during the formation of a chemical bond; 2 - without its formation.

Figure 2.1 shows an energy curve characteristic of interacting hydrogen atoms. The approach of atoms is accompanied by the release of energy, which will be the greater, the more the electron clouds overlap. However, under normal conditions, due to the Coulomb repulsion, it is impossible to achieve the fusion of the nuclei of two atoms. This means that at some distance, instead of attracting atoms, they will repulse. Thus, the distance between atoms r0, which corresponds to the minimum on the energy curve, will correspond to the chemical bond length (curve 1). If the electron spins of the interacting hydrogen atoms are the same, then they will repulse (curve 2). The binding energy for various atoms varies within 170–420 kJ/mol (40–100 kcal/mol).

The process of transition of an electron to a higher energy sublevel or level (i.e., the process of excitation or depairing, which was mentioned earlier) requires the expenditure of energy. When a chemical bond is formed, energy is released. In order for the chemical bond to be stable, it is necessary that the increase in the energy of the atom due to excitation be less than the energy of the formed chemical bond. In other words, it is necessary that the energy expended on the excitation of atoms be compensated by the release of energy due to the formation of a bond.

A chemical bond, in addition to the bond energy, is characterized by length, multiplicity and polarity. For a molecule consisting of more than two atoms, the angles between the bonds and the polarity of the molecule as a whole are significant.

The bond multiplicity is determined by the number of electron pairs that bind two atoms. So, in ethane, H3C–CH3, the bond between carbon atoms is single, in ethylene, H2C=CH2, it is double, and in acetylene, HCºCH, it is triple. As the bond multiplicity increases, the binding energy increases: the C–C bond energy is 339 kJ/mol, C=C - 611 kJ/mol, and CºC - 833 kJ/mol.

The chemical bond between atoms is due to the overlap of electron clouds. If the overlap occurs along the line connecting the nuclei of atoms, then such a bond is called a sigma bond (σ bond). It can be formed by two s-electrons, s- and p-electrons, two px-electrons, s and d electrons (for example

):

A chemical bond carried out by one electron pair is called a single bond. A single bond is always a σ-bond. Orbitals of type s can only form σ bonds.

The bond of two atoms can be carried out by more than one pair of electrons. Such a connection is called a multiple. An example of the formation of a multiple bond is the nitrogen molecule. In the nitrogen molecule, the px orbitals form one σ bond. When a bond is formed by pz orbitals, two regions arise

overlaps - above and below the x-axis:

Such a connection is called a pi-bond (π-bond). The emergence of a π-bond between two atoms occurs only when they are already connected by a σ-bond. The second π-bond in the nitrogen molecule is formed by the py-orbitals of the atoms. When π-bonds are formed, the electron clouds overlap less than in the case of σ-bonds. As a result, π bonds are usually less strong than σ bonds formed by the same atomic orbitals.

p-orbitals can form both σ- and π-bonds; in multiple bonds, one of them is necessarily a σ-bond:

.

Thus, in a nitrogen molecule, out of three bonds, one is a σ-bond and two are π-bonds.

The bond length is the distance between the nuclei of bonded atoms. The bond lengths in various compounds are tenths of a nanometer. As the multiplicity increases, the bond lengths decrease: the N–N, N=N, and NºN bond lengths are 0.145; 0.125 and 0.109 nm (10-9 m), and the bond lengths C-C, C=C and CºC are, respectively, 0.154; 0.134 and 0.120 nm.

Between different atoms, a pure covalent bond can manifest itself if the electronegativity (EO) of some molecules is electrosymmetric, i.e. The "centers of gravity" of the positive charges of the nuclei and the negative charges of the electrons coincide at one point, therefore they are called non-polar.

If the connecting atoms have different EC, then the electron cloud located between them shifts from a symmetrical position closer to the atom with a higher EC:

The displacement of the electron cloud is called polarization. As a result of one-sided polarization, the centers of gravity of positive and negative charges in the molecule do not coincide at one point, a certain distance (l) appears between them. Such molecules are called polar or dipoles, and the bond between the atoms in them is called polar.

A polar bond is a kind of covalent bond that has undergone a slight one-sided polarization. The distance between the "centers of gravity" of positive and negative charges in a molecule is called the dipole length. Naturally, the greater the polarization, the greater the length of the dipole and the greater the polarity of the molecules. To assess the polarity of molecules, a permanent dipole moment (Mp) is usually used, which is the product of the elementary electric charge (e) and the dipole length (l), i.e.

.

Chemical bond.

determination of a chemical bond;

types of chemical bonds;

method of valence bonds;

the main characteristics of the covalent bond;

mechanisms for the formation of a covalent bond;

complex compounds;

molecular orbital method;

intermolecular interactions.

CHEMICAL BOND DETERMINATION

chemical bond called the interaction between atoms, leading to the formation of molecules or ions and the strong holding of atoms near each other.

The chemical bond has an electronic nature, that is, it is carried out due to the interaction of valence electrons. Depending on the distribution of valence electrons in a molecule, the following types of bonds are distinguished: ionic, covalent, metallic, etc. An ionic bond can be considered as the limiting case of a covalent bond between atoms that differ sharply in nature.

TYPES OF CHEMICAL BOND

Ionic bond.

The main provisions of the modern theory of ionic bonding.

An ionic bond is formed during the interaction of elements that differ sharply from each other in properties, that is, between metals and non-metals.

The formation of a chemical bond is explained by the striving of atoms to achieve a stable eight-electron outer shell (s 2 p 6).

Ca: 1s 2 2s 2p 6 3s 2p 6 4s 2

Ca 2+ : 1s 2 2s 2 p 6 3s 2 p 6

Cl: 1s 2 2s 2p 6 3s 2p 5

Cl–: 1s 2 2s 2 p 6 3s 2 p 6

The formed oppositely charged ions are held near each other due to electrostatic attraction.

The ionic bond is not directional.

There is no pure ionic bond. Since the ionization energy is greater than the energy of electron affinity, the complete transition of electrons does not occur even in the case of a pair of atoms with a large difference in electronegativity. Therefore, we can talk about the share of ionicity of the bond. The highest bond ionicity occurs in fluorides and chlorides of s-elements. Thus, in RbCl, KCl, NaCl, and NaF crystals, it is 99, 98, 90, and 97%, respectively.

covalent bond.

The main provisions of the modern theory of covalent bonds.

A covalent bond is formed between elements that are similar in properties, that is, non-metals.

Each element provides 1 electron for the formation of bonds, and the spins of the electrons must be antiparallel.

If a covalent bond is formed by atoms of the same element, then this bond is not polar, that is, the common electron pair is not shifted to any of the atoms. If the covalent bond is formed by two different atoms, then the common electron pair is shifted to the most electronegative atom, this polar covalent bond.

When a covalent bond is formed, the electron clouds of the interacting atoms overlap, as a result, a zone of increased electron density appears in the space between the atoms, which attracts the positively charged nuclei of the interacting atoms and holds them near each other. As a result, the energy of the system decreases (Fig. 14). However, with a very strong approach of atoms, the repulsion of the nuclei increases. Therefore, there is an optimal distance between the nuclei ( bond length,l at which the system has the minimum energy. In this state, energy is released, called binding energy - E St.

Rice. Fig. 14. Dependence of the energy of systems of two hydrogen atoms with parallel (1) and antiparallel (2) spins on the distance between the nuclei (E is the energy of the system, Eb is the binding energy, r is the distance between the nuclei, l is the bond length).

Two methods are used to describe a covalent bond: the valence bond method (BC) and the molecular orbital method (MMO).

VALENCE BOND METHOD.

The VS method is based on the following provisions:

1. A covalent chemical bond is formed by two electrons with oppositely directed spins, and this electron pair belongs to two atoms. Combinations of such two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valent schemes.

2. The stronger the covalent bond, the more the interacting electron clouds overlap.

For a visual representation of valence schemes, the following method is usually used: electrons located in the outer electronic layer are denoted by dots located around the chemical symbol of the atom. The electrons common to two atoms are shown by dots placed between their chemical symbols; a double or triple bond is denoted respectively by two or three pairs of common dots:

N:1s2 2s 2 p 3 ;

C:1s2 2s 2 p 4

It can be seen from the above diagrams that each pair of electrons that binds two atoms corresponds to one dash depicting a covalent bond in the structural formulas:

The number of common electron pairs that bind an atom of a given element with other atoms, or, in other words, the number of covalent bonds formed by an atom, is called covalence according to the VS method. So, the covalence of hydrogen is 1, nitrogen - 3.

According to the way the electronic clouds overlap, there are two types of connections:  - connection and  - connection.

 - connection occurs when two electron clouds overlap along the axis connecting the nuclei of atoms.

Rice. 15. Scheme of education  - connections.

 - bond is formed when electron clouds overlap on both sides of the line connecting the nuclei of interacting atoms.

Rice. 16. Scheme of education  - connections.

MAIN CHARACTERISTICS OF COVALENT BOND.

1. Bond length, ℓ. This is the minimum distance between the nuclei of interacting atoms, which corresponds to the most stable state of the system.

2. Bond energy, E min - this is the amount of energy that must be spent to break the chemical bond and to remove atoms from the interaction.

3. Dipole moment of bond, ,=qℓ. The dipole moment serves as a quantitative measure of the polarity of a molecule. For nonpolar molecules, the dipole moment is 0, for nonpolar molecules it is not 0. The dipole moment of a polyatomic molecule is equal to the vector sum of the dipoles of individual bonds:

4. A covalent bond is characterized by orientation. The orientation of a covalent bond is determined by the need for maximum overlap in space of electron clouds of interacting atoms, which lead to the formation of the strongest bonds.

Since these -bonds are strictly oriented in space, depending on the composition of the molecule, they can be at a certain angle to each other - such an angle is called a valence angle.

Diatomic molecules have a linear structure. Polyatomic molecules have a more complex configuration. Let us consider the geometry of various molecules using the example of the formation of hydrides.

1. Group VI, main subgroup (except oxygen), H 2 S, H 2 Se, H 2 Te.

S1s 2 2s 2 r 6 3s 2 r 4

For hydrogen, an electron with s-AO participates in the formation of a bond, for sulfur, 3p y and 3p z. The H 2 S molecule has a planar structure with an angle between bonds of 90 0 . .

Fig 17. The structure of the H 2 E molecule

2. Hydrides of elements of the V group, the main subgroup: PH 3, AsH 3, SbH 3.

P 1s 2 2s 2 p 6 3s 2 p 3.

In the formation of bonds take part: in hydrogen s-AO, in phosphorus - p y, p x and p z AO.

The PH 3 molecule has the shape of a trigonal pyramid (at the base is a triangle).

Figure 18. The structure of the EN 3 molecule

5. Saturability covalent bond is the number of covalent bonds that an atom can form. It is limited, because An element has a limited number of valence electrons. The maximum number of covalent bonds that a given atom can form in the ground or excited state is called its covalency.

Example: hydrogen is monovalent, oxygen is bivalent, nitrogen is trivalent, etc.

Some atoms can increase their covalence in an excited state due to the separation of paired electrons.

Example. Be 0 1s 2 2s 2

A beryllium atom in an excited state has one valence electron in 2p-AO and one electron in 2s-AO, that is, covalence Be 0 = 0 and covalence Be * = 2. During the interaction, hybridization of orbitals occurs.

Hybridization- this is the alignment of the energy of various AO as a result of mixing before chemical interaction. Hybridization is a conditional technique that makes it possible to predict the structure of a molecule using a combination of AOs. Those AOs whose energies are close can take part in hybridization.

Each type of hybridization corresponds to a certain geometric shape of the molecules.

In the case of hydrides of elements of group II of the main subgroup, two identical sp-hybrid orbitals participate in the formation of the bond. This type of bond is called sp hybridization.

Fig. 19. VeH 2 .sp-hybridization molecule.

sp-hybrid orbitals have an asymmetric shape, elongated parts of the AO with a bond angle of 180 o are directed towards hydrogen. Therefore, the BeH 2 molecule has a linear structure (Fig.).

Let us consider the structure of hydride molecules of elements of group III of the main subgroup using the example of the formation of a BH 3 molecule.

B 0 1s 2 2s 2 p 1

Covalence B 0 = 1, covalency B * = 3.

Three sp-hybrid orbitals take part in the formation of bonds, which are formed as a result of the redistribution of electron densities s-AO and two p-AO. This type of connection is called sp 2 - hybridization. The bond angle at sp 2 - hybridization is equal to 120 0, therefore, the BH 3 molecule has a flat triangular structure.

Fig.20. BH 3 molecule. sp 2 -Hybridization.

Using the example of the formation of a CH 4 molecule, let us consider the structure of the hydride molecules of elements of group IV of the main subgroup.

C 0 1s 2 2s 2 p 2

Covalence C 0 = 2, covalency C * = 4.

In carbon, four sp-hybrid orbitals are involved in the formation of a chemical bond, formed as a result of the redistribution of electron densities between s-AO and three p-AO. The shape of the CH 4 molecule is a tetrahedron, the bond angle is 109 o 28`.

Rice. 21. Molecule CH 4 .sp 3 -Hybridization.

Exceptions to the general rule are H 2 O and NH 3 molecules.

In a water molecule, the angles between bonds are 104.5 o. Unlike hydrides of other elements of this group, water has special properties, it is polar, diamagnetic. All this is explained by the fact that in the water molecule the bond type is sp 3 . That is, four sp - hybrid orbitals are involved in the formation of a chemical bond. Two orbitals contain one electron each, these orbitals interact with hydrogen, the other two orbitals contain a pair of electrons. The presence of these two orbitals explains the unique properties of water.

In the ammonia molecule, the angles between the bonds are approximately 107.3 o, that is, the shape of the ammonia molecule is a tetrahedron, the bond type is sp 3 . Four hybrid sp 3 orbitals take part in the formation of a bond in a nitrogen molecule. Three orbitals contain one electron each, these orbitals are associated with hydrogen, the fourth AO contains an unshared pair of electrons, which determines the uniqueness of the ammonia molecule.

MECHANISMS OF COVALENT BOND FORMATION.

MVS makes it possible to distinguish three mechanisms for the formation of a covalent bond: exchange, donor-acceptor, and dative.

exchange mechanism. It includes those cases of the formation of a chemical bond, when each of the two bonded atoms allocates one electron for socialization, as if exchanging them. To bind the nuclei of two atoms, the electrons must be in the space between the nuclei. This area in the molecule is called the binding area (the area where the electron pair is most likely to stay in the molecule). In order for the exchange of unpaired electrons in atoms to occur, the overlap of atomic orbitals is necessary (Fig. 10.11). This is the action of the exchange mechanism for the formation of a covalent chemical bond. Atomic orbitals can overlap only if they have the same symmetry properties about the internuclear axis (Fig. 10, 11, 22).

Rice. 22. AO overlap that does not lead to the formation of a chemical bond.

Donor-acceptor and dative mechanisms.

The donor-acceptor mechanism is associated with the transfer of a lone pair of electrons from one atom to a vacant atomic orbital of another atom. For example, the formation of an ion -:

The vacant p-AO in the boron atom in the BF 3 molecule accepts a pair of electrons from the fluoride ion (donor). In the resulting anion, four B-F covalent bonds are equivalent in length and energy. In the original molecule, all three B–F bonds were formed by the exchange mechanism.

Atoms, the outer shell of which consists only of s- or p-electrons, can be either donors or acceptors of the lone pair of electrons. Atoms that have valence electrons also on d-AO can simultaneously act as both donors and acceptors. To distinguish between these two mechanisms, the concepts of the dative mechanism of bond formation were introduced.

The simplest example of a dative mechanism is the interaction of two chlorine atoms.

Two chlorine atoms in a chlorine molecule form an exchange covalent bond by combining their unpaired 3p electrons. In addition, the Cl-1 atom transfers the lone pair of electrons 3p 5 - AO to the Cl- 2 atom to the vacant 3d-AO, and the Cl- 2 atom transfers the same pair of electrons to the vacant 3d-AO of the Cl- 1 atom. Each atom simultaneously performs the functions of an acceptor and a donor. This is the dative mechanism. The action of the dative mechanism increases the strength of the bond, so the chlorine molecule is stronger than the fluorine molecule.

COMPLEX CONNECTIONS.

According to the principle of the donor-acceptor mechanism, a huge class of complex chemical compounds is formed - complex compounds.

Complex compounds are compounds that have in their composition complex ions capable of existing both in crystalline form and in solution, including a central ion or atom associated with negatively charged ions or neutral molecules by covalent bonds formed by the donor-acceptor mechanism.

The structure of complex compounds according to Werner.

Complex compounds consist of an inner sphere (complex ion) and an outer sphere. The connection between the ions of the inner sphere is carried out according to the donor-acceptor mechanism. Acceptors are called complexing agents, they can often be positive metal ions (except for metals of the IA group) that have vacant orbitals. The ability to complex formation increases with an increase in the charge of the ion and a decrease in its size.

Donors of an electron pair are called ligands or addends. Ligands are neutral molecules or negatively charged ions. The number of ligands is determined by the coordination number of the complexing agent, which, as a rule, is equal to twice the valency of the complexing ion. Ligands are either monodentate or polydentate. The dentancy of a ligand is determined by the number of coordination sites that the ligand occupies in the coordination sphere of the complexing agent. For example, F - - monodentate ligand, S 2 O 3 2- - bidentate ligand. The charge of the inner sphere is equal to the algebraic sum of the charges of its constituent ions. If the inner sphere has a negative charge, it is an anionic complex; if it is positive, it is a cationic complex. Cationic complexes are called by the name of the complexing ion in Russian, in anionic complexes the complexing agent is called in Latin with the addition of the suffix - at. The connection between the outer and inner spheres in a complex compound is ionic.

Example: K 2 - potassium tetrahydroxozincate, an anionic complex.

2- - inner sphere

2K+ - outer sphere

Zn 2+ - complexing agent

OH - - ligands

coordination number - 4

the connection between the outer and inner spheres is ionic:

K 2 \u003d 2K + + 2-.

the bond between the Zn 2+ ion and hydroxyl groups is covalent, formed by the donor-acceptor mechanism: OH - - donors, Zn 2+ - acceptor.

Zn 0: … 3d 10 4s 2

Zn 2+ : … 3d 10 4s 0 p 0 d 0

Types of complex compounds:

1. Ammonia - ligands of the ammonia molecule.

Cl 2 - tetraamminecopper (II) chloride. Ammonia is obtained by the action of ammonia on compounds containing a complexing agent.

2. Hydroxo compounds - OH - ligands.

Na is sodium tetrahydroxoaluminate. Hydroxo complexes are obtained by the action of an excess of alkali on metal hydroxides, which have amphoteric properties.

3. Aquacomplexes - ligands of the water molecule.

Cl 3 is hexaaquachromium (III) chloride. Aquacomplexes are obtained by the interaction of anhydrous salts with water.

4. Acido complexes - ligands anions of acids - Cl -, F -, CN -, SO 3 2-, I -, NO 2 -, C 2 O 4 - and others.

K 4 - potassium hexacyanoferrate (II). Obtained by the interaction of an excess of a salt containing a ligand on a salt containing a complexing agent.

MOLECULAR ORBITAL METHOD.

MVS quite well explains the formation and structure of many molecules, but this method is not universal. For example, the method of valence bonds does not give a satisfactory explanation for the existence of the ion
, although at the end of the 19th century the existence of a fairly strong molecular hydrogen ion was established
: bond breaking energy here is 2.65 eV. However, no electron pair can be formed in this case, since the composition of the ion
only one electron is included.

The molecular orbital method (MMO) makes it possible to explain a number of contradictions that cannot be explained using the valence bond method.

Basic provisions of the IMO.

When two atomic orbitals interact, two molecular orbitals are formed. Accordingly, when n-atomic orbitals interact, n-molecular orbitals are formed.

Electrons in a molecule belong equally to all the nuclei of the molecule.

Of the two molecular orbitals formed, one has a lower energy than the original, is the bonding molecular orbital, the other has a higher energy than the original, it is antibonding molecular orbital.

MMOs use energy diagrams without scale.

When filling energy sublevels with electrons, the same rules are used as for atomic orbitals:

the principle of minimum energy, i.e. sublevels with lower energy are filled first;

the Pauli principle: at each energy sublevel there cannot be more than two electrons with antiparallel spins;

Hund's rule: the energy sublevels are filled in such a way that the total spin is maximum.

Communication multiplicity. Communication multiplicity in IMO is determined by the formula:

when K p = 0, no bond is formed.

Examples.

1. Can an H 2 molecule exist?

Rice. 23. Scheme of the formation of the hydrogen molecule H 2 .

Conclusion: the H 2 molecule will exist, since the multiplicity of the bond Kp\u003e 0.

2. Can a He 2 molecule exist?

Rice. 24. Scheme of formation of the helium molecule He 2 .

Conclusion: the He 2 molecule will not exist, since the bond multiplicity Kp = 0.

3. Can a particle H 2 + exist?

Rice. 25. Scheme of the formation of the H 2 + particle.

The H 2 + particle can exist, since the multiplicity of the bond Kp > 0.

4. Can an O 2 molecule exist?

Rice. 26. Scheme of the formation of the O 2 molecule.

The O 2 molecule exists. It follows from Fig. 26 that the oxygen molecule has two unpaired electrons. Due to these two electrons, the oxygen molecule is paramagnetic.

Thus the method of molecular orbitals explains the magnetic properties of molecules.

INTERMOLECULAR INTERACTION.

All intermolecular interactions can be divided into two groups: universal and specific. Universal ones appear in all molecules without exception. These interactions are often called connection or van der Waals forces. Although these forces are weak (the energy does not exceed eight kJ/mol), they are the cause of the transition of most substances from the gaseous state to the liquid state, the adsorption of gases by the surfaces of solids, and other phenomena. The nature of these forces is electrostatic.

The main forces of interaction:

1). Dipole - dipole (orientation) interaction exists between polar molecules.

The orientational interaction is the greater, the larger the dipole moments, the smaller the distance between the molecules, and the lower the temperature. Therefore, the greater the energy of this interaction, the higher the temperature to which the substance must be heated in order for it to boil.

2). Inductive interaction occurs when there is contact between polar and non-polar molecules in a substance. A dipole is induced in a nonpolar molecule as a result of interaction with a polar molecule.

Cl  + - Cl  - … Al  + Cl  - 3

The energy of this interaction increases with an increase in the polarizability of molecules, that is, the ability of molecules to form a dipole under the influence of an electric field. The energy of the inductive interaction is much less than the energy of the dipole-dipole interaction.

3). Dispersion interaction- this is the interaction of non-polar molecules due to instantaneous dipoles that arise due to fluctuations in the electron density in atoms.

In a series of substances of the same type, the dispersion interaction increases with an increase in the size of the atoms that make up the molecules of these substances.

4) repulsive forces are due to the interaction of electron clouds of molecules and appear when they are further approached.

Specific intermolecular interactions include all types of donor-acceptor interactions, that is, those associated with the transfer of electrons from one molecule to another. The resulting intermolecular bond has all the characteristic features of a covalent bond: saturation and directionality.

A chemical bond formed by a positively polarized hydrogen that is part of a polar group or molecule and an electronegative atom of another or the same molecule is called a hydrogen bond. For example, water molecules can be represented as follows:

Solid lines are polar covalent bonds inside water molecules between hydrogen and oxygen atoms; dots indicate hydrogen bonds. The reason for the formation of hydrogen bonds is that hydrogen atoms are practically devoid of electron shells: their only electrons are displaced to the oxygen atoms of their molecules. This allows protons, unlike other cations, to approach the nuclei of oxygen atoms of neighboring molecules without experiencing repulsion from the electron shells of oxygen atoms.

The hydrogen bond is characterized by a binding energy of 10 to 40 kJ/mol. However, this energy is sufficient to cause association of molecules those. their association into dimers or polymers, which in some cases exist not only in the liquid state of a substance, but are also preserved when it passes into vapor.

For example, hydrogen fluoride in the gas phase exists as a dimer.

In complex organic molecules, there are both intermolecular hydrogen bonds and intramolecular hydrogen bonds.

Molecules with intramolecular hydrogen bonds cannot enter into intermolecular hydrogen bonds. Therefore, substances with such bonds do not form associates, are more volatile, have lower viscosities, melting and boiling points than their isomers capable of forming intermolecular hydrogen bonds.