Quasirelativstic and relativistic (four-component) versions of the CNDO (Complete Neglect of Differential Overlap) methods have been used in studying the electronic structure of octahedral2[CuF6]4-,2[AuF6]4- complexes andm[Cu6]q,m[Au6]q andm[Au6(PH3)6]q clusters for various charges, q, and spin multiplicities, m. A strong spin-orbit splitting of levels t1u in [Au6]2+ cluster removes the degeneracy of the ground electronic state3T1g into a nondegenerate state so that the Jahn-Teller instability disappears as a consequence of the relativistic effect. The phosphine ligands change the redox stability of the cluster as the orbital energies are shifted to higher values. On the contrary, the spin-orbit splitting of completely filled t2u levels in [AuF6]4- is irrelevant since the degeneracy of the ground electronic state2Eg (Γ8g in the double group notation) remains unchanged. Consequently the Jahn-Teller instability of the octahedral geometry exists and thus a considerable tetragonal distortion appears. © 1990 Academia, Publishing House of the Czechoslovak Academy of Sciences.
