A DENSITY-FUNCTIONAL AND THERMOCHEMICAL STUDY OF M-X BOND LENGTHS ANDENERGIES IN [MX6](2-) COMPLEXES - LDA VERSUS BECKE88 PERDEW86 GRADIENT-CORRECTED FUNCTIONALS/

Local density approximation (LDA) and Becke88/Perdew86 gradient-correc ted density functional theory calculations are used to estimate the he terolytic bond energy, E(M-X), corresponding to the process [MX6](g)(2 -) --> M-g(4+) + 6X(g)(-). The computed data, including scalar relativ istic corrections for second- and third-row metals, are benchmarked ag ainst updated values for the bonds Zr-Cl, Mo-Cl, Pd-CI, Sn-Cl, Hf-Cl, W-Cl, W-Br, Re-Cl, Re-Br, Os-Cl, Ir-CI, Pt-CI, Pt-Br, Ti-Cl, Ti-Br, an d Ni-F derived from a combination of thermochemical and computational data on the antifluorite A(2)MX(6) hexahalometallate(IV) salts. The LD A tends to overbind, and the bond energies are generally too large. Th e BP method systematically reduces these values by about 60 kJ mol(-1) , giving a significantly better comparison with experiment. However, L DA-optimized M-X bond lengths, both in vacuo and for a model 'in cryst al' {K-8[PdCl6]}(6+) cluster, are generally in better agreement with e xperiment.