Gas phase kinetics of neutral transition metal atoms: Reactions of Hf, Ta, Ir, Pt, and Au with alkanes and alkenes

The chemical kinetics of the ground-state, neutral transition metal atoms Hf, Ta, Ir, Pt, and Au with alkanes and alkenes is surveyed. Laser-induced fluorescence measures effective bimolecular rate constants at 300 K in 0.5-1.1 Torr of He buffer gas. Pt(5d(9)6s(1)) is thus far the only ground-state neutral atom in the entire transition metal block that reacts with methane. It also reacts rapidly with linear alkanes, cyclopropane, and the alkenes. Au(5d(10)6s(1)) reacts only with the largest alkenes studied, and then very slowly. In spite of their 6s(2) ground-state configurations, Hf(5d(2)6s(2)), Ta(5d(3)6s(2)), and Ir(5d(7)6s(2)) react rapidly with alkenes including ethene; these atoms do not react with methane and the linear alkanes. An attempt is made to interpret the pattern of reactivity in terms of simple chemical bonding concepts, including electron configuration and spin, guided by earlier extensive electronic structure calculations for the 4d series. It appears that the lanthanide contraction of the 6s orbital strongly influences the chemical reactivity of the 5d-series atoms.