Kinetics of dissociation of H2 from transition-metal polyhydride complexes

Abstract

The kinetics of dissociation of H₂ was determined for the polyhydride complexes [RuH₃(PPh₃)₃]^–, [RuH₄(PPh₃)₃]⁺, [RuH₅(PPh₃)₃]⁺, [OsH₄{P(C₆H₄Me-p)₃}₃] and [OsH₅{P(C₆H₄Me-p)₃}₃]⁺. Dissociation of H₂ from [OsH₃{P(C₆H₄Me-p)₃}₃]^– was too slow to measure up to the decomposition temperature (105 °C). The rates of dissociation increased significantly with increasing protonation and decreased in going from a given ruthenium complex to the corresponding osmium complex. At 105 °C the rate constants span a range of greater than 10¹¹ from 1 × 10⁵ s^–1 for [RuH₅(PPh₃)₃]⁺ to <1 × 10^–6 s^–1 for [OsH₃{P(C₆H₄Me-p)₃}₃]^–. The trend of rate constants is dominated by the variation in ΔH^‡ which ranges from 8.8 to 25.2 kcal mol^–1. The increase in the rate of dissociation of H₂ with increasing protonation does not correlate with a transition from ‘classical’ to ‘non-classical’(i.e.η²-H₂ containing) structures.