Lewis acidity and reactivity of transition metal oxo complexes. A comparative density functional study of CH₃ReO₃, CH₃TcO₃, and their base adducts

A density functional investigation of methyltrioxo-rhenium and its lower homologue methyltrioxo-technetium studying the influence of relativistic 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 distance of 2.50 Å (exp 2.40-2.50 Å). 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 Å) and lower stretching frequencies for the M-N bond in line with the fairly small association energies. From the calculated NH₃ adduct formation energies supported by an analysis of the charge distribution, one concludes that Re is a somewhat stronger Lewis acid center than Tc in the MO₃ moiety. The polarizability (hardness) of the MO₃ fragment is quantified by the HOMO-LUMO splitting as well as by the M-O displacement derivatives of the partial charges and of the dipole moment. These criteria indicate that the reactive MO₃ site of methyltrioxo-rhenium, a model for other electron-deficient early transition metal oxo compounds, is less polarizable and thus will react preferentially with a hard Lewis base such as NH₃ to adducts or with H₂O₂ to the catalytically active peroxo species. The softer Lewis acidic technetium analogue shows a polarizability similar to that of OsO₄ or of late transition metal complexes and thereby is able to directly attack soft Lewis bases, such as olefins. In this way, the fundamental reactivity differences of methyltrioxo-rhenium and osmium tetraoxide can be rationalized.