DENSITY-FUNCTIONAL STUDY OF THE GROUND-SPIN AND EXCITED-SPIN STATES OF [M(2)CL(9)](3-) (M=MO OR W) FACE-SHARED DIMERS - CONSEQUENCES FOR STRUCTURAL VARIATION IN A(3)M(2)CL(9) COMPLEXES

The optimized geometries and relative energies for the ground- and exc ited-spin states (S-max = 0-3) of [M(2)Cl(9)](3-) (M = Mo or W) have b een determined from density-functional calculations. For both systems the calculations predict a dramatic increase in metal-metal distance, with a corresponding increase in M-Cl-b-M bridge angle, as the dimer s pin state (S-max) increases. The terminal MCl(3) groups on the other h and are relatively insensitive to changes in the M-M separation. For b oth [Mo2Cl9](3-) and [W2Cl9](3-) the spin-singlet structure (S-max = 0 ) is predicted to be the most stable species when using the local-dens ity approximation (LDA), in agreement with experiment. In contrast, wh en non-local gradient corrections to the total energy are incorporated , both the spin-quintet (S-max = 2) and -septet(S-max = 3) species are predicted to be more stable than the spin singlet for [Mo2Cl9](3-) Th e calculated (LDA) singlet geometry for [W2Cl9](3-) is in very good ag reement with the observed structure whereas for [Mo2Cl9](3-) the geome try of the spin-triplet species is closer to experiment. Incorporation of relativistic effects is more significant for [W2Cl9](3-) resulting in a further destabilization of the higher-spin states, particularly the spin-quintet and -septet species, relative to the singlet configur ation. Fragment analysis showed that the metal-metal and metal-bridge contributions to the total bonding in the higher-spin species countera ct each other. The destabilization due to loss of metal-metal bonding in the higher-spin states is greater than the stabilization gained fro m the enhanced metal-bridge interaction. However, the reduction in the M-M interaction is more pronounced for [W2Cl9](3-) and thus its highe r-spin states are less accessible than for [Mo2Cl9](3-) accounting for the more dramatic variation in M-M distances observed in A(3)Mo(2)Cl( 9) complexes.