Kinetics of dissociation of H2 from transition-metal polyhydride complexes

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.