Spin-orbit coupling in the tetroxo anions of the d⁰ transition metals and the identification of their lower triplet states

Abstract

A survey is presented of the lower excited states of the tetrahedral oxy-anions having a d⁰ transition metal at the centre, with particular emphasis on the triplet states. The principal question addressed concerns the spin-orbit coupling mechanism in these ions. It appears that the symmetry of the molecular orbitals involved in the first excitation is such that—at least for the 3d compounds VO^3- ₄, CrO^2- ₄, MnO⁻ ₄—the major spin-orbit coupling should occur on the oxygen ligands. Two quantitative problems are considered: the radiative lifetime of the ³T_t state, and the zero-field splitting of this multiplet if spin-orbit coupling is the dominant interaction determining the splitting. The results of the theoretical analysis are compared with recent information on the luminescent triplet states of K₂Cr₂O₇, YVO₄ and CaMoO₄ obtained by E.P.R. The radiative lifetime of 4 ms calculated for VO³⁼ ₄ is in excellent agreement with the scarce experimental information. The spin orbit coupling, however, does not appear to be the dominant interaction determining the multiplet splitting of the 3d and 4d compounds. The tetrahedral ions, in accordance with the Jahn-Teller effect, suffer a distortion on excitation and a single orbital state lies lowest with a fine structure characteristic for a situation of low symmetry.