Andrei L Tchougreeff
Moscow State University, Russie
We readdress the well-known nepheloxetic effect in coordination compounds of
transition-metal ions and analyze its possible origins presented in the
literature. The initial hypothesis was to ascribe the reduction of the
effective Racah parameters B and C of the electron-electron interaction
in the complexes as compared to their respective free ion values (which is
the essence of the nepheloxetic effect) to expansion of d-orbitals in the
complex due to their quantum-mechanical mixing with the orbitals of the
ligands leading to delocalization. This picture necessarily leads to a rigid
positive correlation between the amount of the d-shell splitting
controlled by the same delocalization and the amount of the renormalization
of the interaction parameters. In fact, such a rigid relation does not exist
and the so called spectrochemical and nepheloxetic series of the ligands
composed according to the above amounts do not coincide in many points.
An
alternative explanation based not on the delocalization, but on polarization
of the ligands had been proposed about the same time as the delocalization
based one. Realistic estimates had been obtained on this basis, but the
scheme had never been implemented on the atomic scale which is necessary to
enable renormalization of the above interaction parameters as a "built-in"
function of quantum chemical software.
The required atomic resolution
formulation of the polarization based model of the nepheloxetic effect is
constructed, where also a relation of such an approach
to the general problematics of the "next generation" of semiempirical
methods of quantum chemistry is discussed.