Bonding considerations ligand field theory

The successful rationalization of these transition-metal inverse spinel structures in terms of the relative LFSE s of tetrahedral and octahedral sites is another attractive vindication of ligand-field theory as applied to structure and thermodynamic properties. Once again, however, we must be very careful not to extrapolate this success. Thus, we have a clear prediction that LSFE contributions favour tetrahedral over octahedral coordination, except for d" with n = 0, 5 or 10. We do not expect to rationalize the relative paucity of tetrahedral nickel(ii) species relative to octahedral ones on this basis, however. Many factors contribute to this, the most obvious and important one being the greater stabilization engendered by the formation of six bonds in octahedral species relative to only four bonds in tetrahedral ones. Compared with that, the differences in LSFE s is small beer. Why , one asks, was our rationalization of spinel structures so successful when we neglected to include consideration of the bond count The answer is that cancellations within the extended lattice of the spinels tend to diminish the importance of this term. 

The analogy between the two theories is only formal. Crystal field theory is a purely electrostatic approach that does not take into consideration the formation of MOs and the nature of the bond. According to crystal field theory, optical and magnetic properties are ascribed to crystal field splitting between two AOs, whereas in ligand field theory energy splitting occurs between AOs, and... 

A holistic molecular orbital theory description of bonding in complexes provides a more sophisticated model of bonding in complexes, leading to ligand field theory (LFT), which deals better with ligand influences. Both CFT and LFT reduce to equivalent consideration of d electron location in a set of five core d orbitals. 

The structure of arene sandwich compounds (like that of cyclopentadienyl, cycloheptatrienyl, etc., derivatives) and the character of the M-arene bond may also be considered on the basis of ligand field theory assuming C ,v symmetry of the compound (cf. Chapter 9). The ligands connected to the metal via carbon atoms create a strong field. Therefore, the splitting parameters are large. The nephelauxetic coefficients P = BIBq, where B is the Racah parameter for the complex and Bq is the Racah parameter for the free metal ion (5o and B represent interelectron repulsion), generally assume values 0.5 0.1 for metallocenes and arene sandwich complexes. Low values of nephelauxetic coefficients indicate considerable covalent character of the metal-hydrocarbon bond. 

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