Coordination compounds octahedral fields

MOMEC is a force field for describing transition metal coordination compounds. It was originally parameterized to use four valence terms, but not an electrostatic term. The metal-ligand interactions consist of a bond-stretch term only. The coordination sphere is maintained by nonbond interactions between ligands. MOMEC generally works reasonably well for octahedrally coordinated compounds. 

Note that a combination of various types of potentials can be activated. For example, the coordination geometry for octahedral transition metal compounds can be modeled by 1,3-nonbonded interactions in combination with a multiple harmonic function. This is the approach used in the MOMEC97 force field for a number of metal complexes. Also, for four-coordinate compounds one or both of these potentials can be combined with a plane twist potential that enforces square-planar geometry. 

The coordination chemistry of Cr can be subdivided into the realm of mononuclear coordination compounds, which can be derived (at least formally) from the blue [Cr(OH2)e] ion, and the chemistry of metal-metal bonded species (analogs of Cr2(OAc)4). Most of the former are octahedral high-spin complexes with distinct tetragonal distortions caused by the Jahn - Teller Effect. With strong-field ligands, low-spin octahedral complexes are formed, and there are also some four-coordinate complexes of various coordination geometries. The metal-metal bonded species are described in Section 5. 

Absorption spectra of coordination compounds can be used to determine the magnitude of the ligand field splitting, which is A for octahedral complexes. It should be made clear from the outset that the accuracy with which Ag can be determined is to some extent limited by the mathematical tools used to solve the problem. Absorption spectra often have overlapping bands to determine the positions of the bands accurately, therefore, requires an propriate mathematical technique for reducing overlapping bands into their individual components. Such analysis is beyond the scope of this text. However, we can often obtain A values (and sometimes values of the Racah parameter, B) of reasonable accuracy simply by using the positions of the absorption maxima taken directly from the spectra. 

One of the crucial points in the recent developments of coordination compound photochemistry has been the debate concerning the identity of the excited state(s) responsible for the photosolvation reactions that are obtained by irradiating Cr(III) complexes in their ligand field bands (5,8), Direct photolysis experiments revealed that the most likely candidates (see e.g,. Figures 2 and 3) are the lowest spin-allowed excited state ( T2ff in octahedral symmetry) and the lowest spin-forbidden excited state ( Eg). Such experiments, however, did not warrant any definite conclusion about the actual role of each of these two states (5). 

TABLE 16.15 Summary of the results of ligand field theory and the angular overlap model for octahedral coordination compounds. 

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