Kinetics of dihydrogen activation by Co 2Rh 2(CO) 12 disproportionation and a second order cluster fragmentation

Abstract

The highly reactive mixed metal cluster Co 2Rh 2(CO) 12 has been shown to undergo selective disproportionation under hydrogen according to 3Co 2Rh 2(CO) 12 → 2Co 3Rh(CO) 12+Rh 4(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 Co 2Rh 2(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 Co 2Rh 2(CO) 12 is very unusual for a cluster fragmentation reaction. Possible mechanisms giving rise to the observed kinetics are discussed.