KINETICS OF THE FORMATION OF COBALT RHODIUM HEPTACARBONYL

The kinetics of the reaction Co2Rh2(CO)12 + 2CO --> 2CoRh(CO)7 have been studied at T = 267-294 K and at P(CO) = 0.05-0.20 MPa (8.7 x 10(-4)-4.8 x 10(-3) mole fraction) in n-hexane as solvent. The rate of the reaction was found to be first order in the mixed-metal cluster Co2Rh2(CO)12 (1) and to be consistent with the two-term rate expression rate = {k1 + k2[CO]}[Co2Rh2(CO)12], where the term k1[Co2Rh2(CO)12] represents the single most important contribution to the overall rate of cluster fragmentation under the present reaction conditions. The activation parameters determined from 20 experiments are DELTA-H double-ended-dagger 1 = 60.6 +/- 1.9 kJ/mol (14.5 +/- 0.4 kcal/mol), DELTA-S double-ended-dagger 1 = -80.3 +/- 6.7 J/(mol K) (-19.2 +/- 1.6 cal/(mol K)) and DELTA-H double-ended-dagger 2 = 53.3 +/- 6.6 kJ/mol (12.7 +/- 1.6 kcal/mol), DELTA-S double-ended-dagger 2 = -64.4 +/- 23.5 J/(mol K) (-15.4 +/- 5.6 cal/(mol K)). It is proposed that two independent reaction pathways are available for the transformation of Co2Rh2(CO)12 (1) to CoRh(CO)7 (2) and that these are characterized by (I) an intramolecular activation of Co2Rh2(CO)12 and (II) either (a) the bimolecular reaction of Co2Rh2(CO)12 with CO or (b) the unimolecular reaction of a preequilibrated species {Co2Rh2(CO)13}, as the rate-determining steps.