KINETICS OF MOLECULAR-HYDROGEN ACTIVATION BY COBALTRHODIUM HEPTACARBONYL

The kinetics of the activation of molecular hydrogen by the coordinatively unsaturated mixed-metal carbonyl CoRh(CO)7 (1) were studied at temperatures between 276 and 294 K, hydrogen pressures between 0.05 and 0.20 MPa, and carbon monoxide pressures between 0.20 and 0.80 MPa in n-hexane as solvent. The reactions were run under isothermal and isobaric conditions, and the extent of the reaction was followed by high-pressure in situ infrared spectroscopy. The immediately observable products of the activation of molecular hydrogen were HCo(CO)4 (2) and Rh4(CO)12 (4), but eventually the dinuclear carbonyl Co2(CO)8(5) began to form via recombination of cobalt carbonyl hydrides. Under these conditions, the reaction was found to be first order in 1, first order in hydrogen, and zero order in carbon monoxide. The experimentally determined parameters of activation are £a = 23 ± 2 kJ/mol (5.6 ± 0.5 kcal/mol), AS* = -169 ± 13 J/(mol K) (-40 ± 3 cal/(mol K)), and Mí* = 21 ± 2 kJ/mol (5.0 ± 0.5 kcal/mol). It is concluded on the basis of the orders of reaction that activation occurred through the bimolecular reaction of molecular hydrogen with CoRh(CO)7 without the prior dissociation of a carbonyl ligand. Further, the magnitude of the activation parameters, particularly the low enthalpy of activation, suggests that this activation occurred via oxidative addition at a single metal site, in all probability at the rhodium, generating a transient rhodium dihydride species |(CO)4CoRhH2(CO)3j, followed by elimination of HCo(CO)4 (2). The observed Rh4(CO)12(4) then arises via the rapid recombination of the rhodium carbonyl species (HRh(CO)3¡ (3). © 1990, American Chemical Society. All rights reserved.