LEWIS ACIDITY AND REACTIVITY OF TRANSITION-METAL OXO COMPLEXES - A COMPARATIVE DENSITY-FUNCTIONAL STUDY OF CH3REO3, CH3TCO3, AND THEIR BASEADDUCTS

A density functional investigation of methyltrioxo-rhenium and its low er homologue methyltrioxo-technetium studying the influence of relativ istic effects on the reactivity of the two metal centers is presented. In order to quantify the Lewis acidity of the central metal atom, the adduct formation with NH3 as a probe molecule is investigated. Methyl trioxo-rhenium is calculated to form a stable base adduct at a Re-N di stance of 2.50 Angstrom (exp 2.40-2.50 Angstrom). The calculations for the Tc complex (and for the Re complex at a nonrelativistic level of theory) yield longer M-N distances (by 0.1 Angstrom) and lower stretch ing frequencies for the M-N bond in line with the fairly small associa tion energies, From the calculated NH3 adduct formation energies suppo rted by an analysis of the charge distribution, one concludes that Re is a somewhat stronger Lewis acid center than Tc in the MO3 moiety. Th e polarizability (hardness) of the MO3 fragment is quantified by the H OMO-LUMO splitting as well as by the M-O displacement derivatives of t he partial charges and of the dipole moment. These criteria indicate t hat the reactive MO3, site of methyltrioxo-rhenium, a model for other electron-deficient early transition metal oxo compounds, is less polar izable and thus will react preferentially with a hard Lewis base such as NH3 to adducts or with H2O2 to the catalytically active peroxo spec ies. The softer Lewis acidic technetium analogue shows a polarizabilit y similar to that of OsO4 or of late transition metal complexes and th ereby is able to directly attack soft Lewis bases, such as olefins. In this way, the fundamental reactivity differences of methyltrioxo-rhen ium and osmium tetraoxide can be rationalized.