JAHN-TELLER COUPLING IN TETRAHEDRAL OXO V-IV CR-V AND MN-VI CLUSTERS - AN AB-INITIO STUDY

Multiconfiguration complete active-space calculations have been carrie d our on tetragonally distorted tetrahedral oxo anions MO4z (M, z = V- IV, -4; Cr-v, -3 and Mn-VI, -2) in their d(1) (E-2) ground states. Con tributions of the d(1) configuration to the (2)A(1)(B-2(1)) ground sta tes functions for tetragonally compressed (elongated) geometries decre ase from V-IV to Cr-V and Mn-VI, reflecting a considerable intermixing of ligand-to-metal charge transfer (LMCT) states for Cr-V and Mn-VI. Linear (A(1)) and quadratic (A(2)) Jahn-Teller coupling constants dedu ced from numerical data show that vibronic coupling decreases as the m etal-ligand interactions become more covalent and LMCT stares shift to lower energies from V-IV to Cr-V and Mn-VI. Contrary to data reported previously, A(1) and A(2) for V-IV are large and of opposite sign imp lying absolute minima for tetragonally compressed geometries and a (2) A(1) ground state. Vibronic coupling in Cr-V and Mn-VI is found small for linear (A(1)) and negligible for quadratic (A(2)) terms. This indi cates a moderate to strong coupling for V-IV but a weak one for both C rO43- and MnO42- clusters. The theoretical results compare reasonably with vibronic parameters and E-2 ground state splittings deduced from published EPR and electronic spectra on Cr-V. In the light of these re sults a qualitative explanation of available (but still not well under stood) optical and EPR-data of the V-IV-doped zircon (ZrSiO4) type str uctures is proposed.