WHY IS 10DQ SO USEFUL FOR MEASURING CHANGES OF THE IMPURITY-LIGAND DISTANCE

The microscopic origin of the strong dependence displayed by the cubic field splitting parameter, 10Dq, on the metal-ligand distance in tran sition metal complexes is explored within a realistic molecular orbita l framework. Taking as a guide the case of the CrF63-Oh complex it is shown that through the admixture of 2s(F-) orbitals in the antibonding eg (similar to 3z(2) - r(2);x(2) - y(2)) orbital is very small [(N-e lambda(s))(2) similar or equal to 5%] it is the main responsible of t he law 10Dq proportional to R(-n) with n close to five. By contrast th e admixture of 2p(F-) orbitals though more important [(N-e lambda(p si gma))(2) similar or equal to 16%] is shown to be no relevant as regard s the R dependence of 10Dq. Thus this analysis demonstrates that the R dependence of 10Dq is ultimately related to that displayed by the iso tropic superhyperfine constant for O-h complexes with unpaired sigma-e lectrons.