Crystal field splitting octahedral complex

Figure 20-12 summarizes the electrical interactions of an octahedral complex ion. The three orbitals that are more stable are called 2 g orbitals, and the two less stable orbitals are called Sg orbitals. The difference in energy between the two sets is known as the crystal field splitting energy, symbolized by the Greek letter h. 

More recently it has been found15 that a correlation exists between spectroscopic parameters of the divalent aqua ions of the metals Cr to Ni, and the polarographic y2. A linear relationship was found between A0 and crystal field splitting parameter, ot the transfer coefficient, n the number of electrons transferred in the reduction, EVl the polarographic half-wave potential and E° the standard electrode potential. The use of the crystal field splitting parameter would seem to be a more sensible parameter to use than the position of Amax for the main absorption band as the measured Amax may not be a true estimate of the relevant electronic transition. This arises because the symmetry of the complex is less than octahedral so that the main absorption band in octahedral symmetry is split into at least two components with the result that... 

Figure 1-12. Simplified molecular orbital diagram for the formation of an octahedral ML6 complex in which there are no Tt-bonding interactions between metal and ligand. The labels on the molecular orbitals refer to their symmetries. Notice the central region may be equated to the crystal field splitting of the d orbitals.

However, since CFT is a rather crude model, such an exact relationship is seldom of use. Instead, it is instructive and convenient to realize that, with all conditions being equal, the crystal field splitting for a tetrahedral complex is about half of that for an octahedral complex. 

Crystal field spectral measurements of transition metal ions doped in a variety of silicate glass compositions (e.g., Fox et al., 1982 Nelson et al., 1983 Nelson and White, 1986 Calas and Petiau, 1983 Keppler, 1992) have produced estimates of the crystal field splitting and stabilization energy parameters for several of the transition metal ions, examples of which are summarized in table 8.1. Comparisons with CFSE data for each transition metal ion in octahedral sites in periclase, MgO (divalent cations) and corundum, A1203 (trivalent cations) and hydrated complexes show that CFSE differences between crystal and glass (e.g., basaltic melt) structures,... 

The first-order JT effect is important in complexes of transition metal cations that contain nonuniformly filled degenerate orbitals, if the mechanism is not quenched by spin-orbit (Russell-Saunders) coupling. Thus, the JT effect can be expected with octahedrally coordinated and high spin d cations, and tetrahedrally coordinated and d cations. The low-spin state is not observed in tetrahedral geometry because of the small crystal field splitting. Also, spin-orbit coupling is usually the dominant effect in T states so that the JT effect is not observed with tetrahedrally coordinated d, d , d, and d ions. 

Figure 5 The molecular orbital or ligand field picture of metal-ligand bonding in an MLe complex. Compare this picture with Figure 1 to see how the octahedral crystal field splitting pattern (in the box) is still present in the MO model...

We will briefly consider the crystal field splitting of the rf-orbitals in four-coordinate, tetrahedral complexes. The cube, octahedron and tetrahedron are related geometrically. Octahedral coordination results when ligands are placed in the centers of cube faces, while tetrahedral coordination results when ligands are placed on alternate comers of a cube, as shown in Fig. 10.9. 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

For octahedral complexes, the degenerate d orbitals split into two levels, a set of three lower energy t2g orbitals and a pair of higher energy eg orbitals. The energy difference between the two levels is called the crystal field splitting energy Ao. 

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