SPIN TRANSFER VERSUS CHARGE-TRANSFER IN THE EXCITED-STATES OF 3D-DEGREES TETROXO IONS

The first absorption bands in the optical spectra of the 3d-degrees te troxo-ions, VO4=, CrO4=, MnO4-, are commonly assumed to arise from ele ctron transfer from a t1 ligand MO to an MO centered on the metal. Fir st electron-spin-resonance experiments on the luminescent triplet stat e of VO4= in an YPO4 host are reviewed. These show that one unpaired e lectron indeed resides on the metal; the other is not in a t1-type MO but localized on a single ligand. This uneveness of the spin distribut ion is attributed to a Jahn-Teller instability that deforms the isolat ed ion from T(d) to C3v symmetry. Then ab-initio calculations on CrO4- and CrO4= are considered. The results for the 2E(t1(5)) ground state of CrO4- confirm such a Jahn-Teller effect with the spin localized on a single oxygen (the apex of the C3v pyramid), yet the charge is almos t evenly distributed over the four ligands. For the lowest triplet sta te of CrO4=, with a (t1(5) e) electron configuration, an analogous in stability is predicted. Although, again, an uneven spin distribution o ver the ligands results, owing to relaxation of electron pairs in clos ed shells the charge distribution is practically not changed by the ex citation. Thus, what is commonly labelled a 'charge transfer' transiti on may well correspond to spin transfer without an attendant charge tr ansfer.