Ligand field considerations

Quadrupole splittings are often interpreted from ligand field models with simple rules for the contributions from each occupied f-orbital (see discussion above). However, these models fail even qualitatively in the case of more covalent metal-ligand bonds. An example concerns the quadrupole spUttings of Fe(IV)-oxo sites in their 5 = 1 or 5 = 2 spin states. Here, ligand field considerations do not even provide the correct sign of the quadrupole splitting [60]. 

From the ligand field considerations outlined previously, and the values in Table 5.6, the EFG along the Fe-O bond should be dominated by the one-center valence contributions around the iron and given by ... 

Basic ligand-field considerations predict the following energetic order of valence (i-orbitals for six-coordinate iron-nitrido compounds with approximate 4 sym-... 

If the assignment of kjis, correct, the rate of dissociation must be given by — -1 -2/ 2 - In this composite function the individual rate constants for the back reactions refer to the breaking of M-N bonds, and therefore k should vary markedly with substituents on the ligand (232) further, the variation of k with metal is d > d > d > d < d < d , the order to be expected from ligand field considerations. 

The limiting value, AG kcalmol S had previously been felt to be rather high, and one possible explanation considered was a rapid equilibrium between the two spin states of cobalt(m) tlg t ge prior to the electron transfer. The data from other oxidants have weakened if not altogether removed the necessity for this. It has moreover been argued from ligand-field considerations that for the hexa-aquo-cobalt(iii) ion the spin-change free energy may in any case be quite small. ... 

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