KINETICS OF DIHYDROGEN ACTIVATION BY CO2RH2(CO)12 DISPROPORTIONATION AND A 2ND-ORDER CLUSTER FRAGMENTATION

The highly reactive mixed metal cluster Co2Rh2(CO)12 has been shown to undergo selective disproportionation under hydrogen according to 3Co2Rh2(CO)12 --> 2Co3Rh(CO)12 + Rh4(CO)12. The reaction, which may be classified as an intermolecular redistribution reaction, was carried out in n-hexane at T = 275-294 K and P(H2) = 0.4-1.0 MPa and studied by use of high pressure in situ infrared spectroscopy. No reaction intermediates were observed. The reaction has an order of 1.0 +/- 0.1 with respect to hydrogen and is second order in the metal carbonyl cluster Co2Rh2(CO)12, it has an apparent activation energy E(a) of 55 +/- 8 kJ/mol (13 +/- 2 kcal/mol). The low activation energy suggests that the rate-limiting step may not be the activation of molecular hydrogen. Furthermore, the second order dependence of the rate on the concentration of Co2Rh2(CO)12 is very unusual for a cluster fragmentation reaction. Possible mechanisms giving rise to the observed kinetics are discussed.