According to the degree of increase in the splitting parameter Δ, the ligands are arranged in a row called spectrochemical (Figure 2.9).

Rice. 2.9. Spectrochemical series of ligands

In the interaction of a strong field ligand and CA, splitting occurs d- orbitals. In this case, the distribution of electrons according to Hund's rule becomes impossible, since the transition of electrons from a lower -level to a higher -level requires energy, which is energetically unfavorable (large value of the splitting parameter Δ). Therefore, the electrons first completely fill the -level, and then only the -level is filled. In case of being on d- orbitals of 6 electrons, under the action of a strong field ligand, the -level is filled with pairing of electrons. This creates low spin diamagnetic complex. And in the case of a weak field ligand, when the splitting parameter Δ takes a lower value, a uniform distribution of electrons according to the Hund rule becomes possible. In this case, the pairing of all electrons does not occur; high-spin paramagnetic complex.

The sequence of arrangement of ligands in the spectrochemical series in the framework of MO theory can be explained as follows. The greater the degree of overlap of the initial orbitals, the greater the energy difference between the bonding and loosening orbitals and the greater Δ. In other words, the value of ∆ increases with amplification σ- metal-ligand binding. The value of Δ is also significantly affected by π-bonding between CA and ligands.

If ligands have orbitals (empty or filled) that, according to symmetry conditions, are capable of overlapping with dxy-, dxz- and dyz- CA orbitals, then the MO diagram of the complex becomes much more complicated. In this case, to MO σ- and σ * - type, molecular orbitals π are added - and π* - type. Ligand orbitals capable of π - overlap, for example, p- and d- atomic orbitals or molecular π - and π* - orbitals of binuclear molecules. On Fig. 2.10 shows combinations of ligand orbitals and dxz- CA orbital, which, according to the symmetry conditions, can be combined to form molecular π - orbitals.

Rice. 2.10. dxz- CA orbital (a) and combinations corresponding to it in symmetry p-(b) and π * – (c) ligand orbitals leading to the formation of MO of the octahedral complex

Rice. 2.11. Influence of π - binding by Δ

Participation dxy-, dxz- and dyz- orbitals in the construction of π - orbitals leads to a change in Δ. Depending on the ratio of the energy levels of the CA orbitals and the ligand orbitals combined with them, the value of Δ can increase or decrease (Fig. 2.11).

When π is formed - orbitals of the complex, part of the electron density of CA is transferred to the ligands. Such π - interaction is called dative. When π is formed * - orbitals of the complex, some part of the electron density is transferred from the ligands to the CA. In this case pi - the interaction is called donor-acceptor.

Ligands that are π - acceptors cause more cleavage d- level; ligands that are π - donors, on the contrary, cause a small splitting d- level. The nature σ- and π- ligand interactions can be subdivided into the following groups.

And by John Van Vleck to describe the lower states of transition metal cations surrounded by ligands, both anions and neutral molecules. The theory of the crystal field was further combined [and improved] with the theory of (delocalized) molecular orbitals into a more general one, taking into account the partial covalence of the metal-ligand bond in coordination compounds.

Crystal field theory makes it possible to predict or interpret the optical absorption spectra and electron paramagnetic resonance spectra of crystals and complex compounds, as well as the enthalpies of hydration and stability in solutions of transition metal complexes.

Overview of crystal field theory[ | ]

According to TCP, the interaction between a transition metal and ligands arises due to the attraction between a positively charged metal cation and a negative charge of electrons in the nonbonding orbitals of the ligand. The theory considers the change in the energy of five degenerate d-orbitals surrounded by point charges of ligands. As the ligand approaches the metal ion, the ligand electrons get closer to some d-orbitals than to others, causing a loss of degeneracy. Electrons d-orbitals and ligands repel each other as charges with the same sign. Thus, the energy of those d-electrons that are closer to the ligands become higher than those that are further away, resulting in a splitting of energy levels d-orbitals.

Splitting is influenced by the following factors:

• The nature of the metal ion.
• The degree of oxidation of the metal. The higher the oxidation state, the higher the cleavage energy.
• Location of ligands around a metal ion.
• The nature of the ligands surrounding the metal ion. The stronger the effect of the ligands, the greater the difference between high and low energy levels.

The most common form of ligand coordination is octahedral, at which six ligands create a crystal field of octahedral symmetry around the metal ion. In the octahedral environment of a metal ion with one electron in the outer shell, the d-orbitals are divided into two groups with a difference in energy levels Δ oct ( splitting energy), while the energy of the orbitals dxy, dxz and d yz will be lower than d z 2 and d x 2 -y 2, since the orbitals of the first group are farther from the ligands and experience less repulsion. The three low energy orbitals are denoted as t2g, and two with a high - like e g.

The next most common are tetrahedral complexes in which four ligands form a tetrahedron around a metal ion. In this case d-orbitals are also divided into two groups with a difference in energy levels Δ tetra. In contrast to octahedral coordination, orbitals will have low energy d z 2 and d x 2 -y 2, and high - d xy , d xz and d yz. In addition, since the electrons of the ligands are not directly in the direction d-orbitals, the splitting energy will be lower than with octahedral coordination. With the help of TST, one can also describe flat square and other complex geometries.

The energy level difference Δ between two or more groups of orbitals also depends on the nature of the ligands. Some ligands cause less cleavage than others, for which reasons he explains. Spectrochemical series- empirically obtained list of ligands, ordered in ascending order Δ:

The oxidation state of the metal also affects Δ. A metal with a higher oxidation state attracts ligands closer due to the greater charge difference. Ligands closer to the metal ion cause more cleavage.

Low- and high-spin complexes[ | ]

Large cleavage ligands d-levels, for example CN - and CO, are called ligands strong field. In complexes with such ligands, it is unfavorable for electrons to occupy high-energy orbitals. Therefore, the low energy orbitals are completely filled before the filling of the high energy orbitals begins. Such complexes are called low spin. For example, NO 2 − is a strong field ligand that creates a large splitting. All 5 d-electrons of the octahedral ion 3− will be located at the lower level t 2g .

In contrast, ligands that cause small splitting, such as I − and Br − , are called ligands weak field. In this case, it is easier to put electrons in high energy orbits than it is to put two electrons in the same low energy orbit, because two electrons in one orbit repel each other, and the energy cost of placing a second electron in an orbit is higher than Δ. Thus, before paired electrons appear, in each of the five d-orbitals must be placed one electron at a time in accordance with Hund's rule. Such complexes are called high-spin. For example, Br − is a weak field ligand causing a small splitting. All 5 d-orbitals of the 3− ion, which also has 5 d-electrons will be occupied by one electron.

The splitting energy for tetrahedral complexes Δ tetra is approximately equal to 4/9Δ oct (for the same metal and ligands). As a result, the energy level difference d-orbitals are usually below the electron pairing energy, and tetrahedral complexes are usually high-spin.

Distribution diagrams d-electrons make it possible to predict the magnetic properties of coordination compounds. Complexes with unpaired electrons are paramagnetic and are attracted by a magnetic field, while complexes without them are diamagnetic and weakly repulse.

Crystal Field Stabilization Energy[ | ]

The crystal field stabilization energy (ESF) is the energy of the electronic configuration of a transition metal ion relative to the average energy of the orbitals. Stabilization occurs due to the fact that in the field of ligands the energy level of some orbitals is lower than in a hypothetical spherical field in which all five d-orbitals have the same repulsive force, and all d-orbitals are degenerate. For example, in the octahedral case, the level t2g lower than the average level in a spherical field. Therefore, if there are electrons in these orbitals, then the metal ion is more stable in the ligand field relative to the spherical field. Conversely, the energy level of the orbitals e g above average, and the electrons in them reduce stabilization.

Stabilization energy by an octahedral field

There are three orbitals in an octahedral field t2g stabilized relative to the average energy level by 2/5 Δ oct, and two orbitals e g destabilized by 3/5 Δ oct. Above were examples of two electronic configurations d 5 . In the first example, a low-spin complex 3− with five electrons in t2g. His ESCR is 5 × 2/5 Δ oct = 2Δ oct. In the second example, the high-spin complex 3− with ESCP (3 × 2/5 Δ oct) − (2 × 3/5 Δ oct) = 0. In this case, the stabilizing effect of electrons in low-level orbitals is neutralized by the destabilizing effect of electrons in high-level orbitals.

Diagrams of d-level splitting by a crystal field[ | ]

For the same central ion and the same configuration of the complexes, the greater the splitting parameter A, the stronger the field created by the ligands. The strength of this field is determined by such classical properties of ligands as size, charge, dipole moment (permanent or induced), polarizability, and the ability to form n-bonds. For convenience of consideration, two limiting fields of ligands are distinguished.

Rice. 5.

For weak field ligands, the splitting energy is less than the interelectron repulsion energy.

For strong field ligands, the splitting energy is greater than the interelectron repulsion energy.

The degree of splitting of energy levels by the crystal field is affected by the degree of oxidation of the central atom and the type of (/-electrons) it has. With an increase in the degree of oxidation of the (/-element (increase in the charge of the ion), A increases, since the ligands come closer to the central ion and, therefore, cause a greater splitting of the (/-level. In subgroups of (/-elements, during the transition from the 4th to the 5th and especially to the 6th period, D of complexes of the same type increases noticeably. This is due to the fact that Ad- and 5(/-orbitals extend in space farther from the nucleus than 3(/-orbitals. This corresponds to a stronger repulsion of electrons and ligands and, accordingly, a greater splitting Ad- and 5(/-levels compared to 3(/-level.

Distribution of electrons in d-orbitals. The theory of the crystal field quite simply and clearly explains the magnetic properties of complexes, their spectra, and a number of other properties. To understand these properties, it is necessary to know the nature of the distribution of electrons over the ^/-orbitals of an ion located in the field of ligands. The latter depends on the ratio of the splitting energy D and the repulsion energy.

If the interelectron repulsion energy turns out to be greater than the splitting energy (weak field ligand), then five ^/-orbitals are successively filled, first one by one, and then by the second electron.

If the splitting energy D exceeds the energy of interelectron repulsion (strong field ligand), then first the orbitals with lower energy are completely filled, and then the orbitals with higher energy. According to the ability to cause splitting of the ^/-level, ligands can be arranged in the following row:

This series, called the spectrochemical series, was found as a result of an experimental study of the spectra of complexes and quantum mechanical calculations.

As an example, let us consider the nature of the distribution of the 3c/-electrons of the Co 3+ ion during the formation of octahedral complexes 34 . In the free ion Co 3+ (3 d e) The electrons are arranged like this:

It is calculated that the repulsion energy of electrons of the same orbital for the Co 3+ ion is 251 kJ/mol, the splitting energy of its 3^/-orbitals in the octahedral field of F ions is 156 kJ/mol, and in the field of NH 3 molecules - 265 kJ/mol.

Thus, in the field of the F* ion, the value of A is small; therefore, the number of unpaired electrons in the orbitals of the split Co 3 " levels is the same as in the free ion (Fig. 6).

Rice. 6. Distribution of d-electrons of the Co 3+ ion in octahedral complexes)