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-degrees-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-degrees-C the rate constants span a range of greater than 10(11) from 1 x 10(5) s-1 for [RuH5(PPh3)3]+ to < 1 x 10(-6) s-1 for [OsH3{P(C6H4Me-p)3}3]-. The trend of rate constants is dominated by the variation in DELTA-H double-ended-dagger 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. eta-2-H2 containing) structures.