REACTION-KINETICS OF SOME SUBSTITUTED TETRARHODIUM CARBONYL CLUSTERS

Quantitative kinetic data have been obtained for reactions, in 1,2-dichloroethane at 25°C, of the known cluster Rh4(CO)9(etpb)3 (etpb = P(OCH2)3CEt), and of the new cluster Rh4(CO)10(PCy3)2 (Cy = C6H11), with some P-donor nucleophiles and, for the latter cluster, with AsPh3. Both clusters obey the two term rate equation: kobs = k1 + k2[L] although the value of k1 for Rh4(CO)10(PCy3)2 is only observed for reactions with AsPh3 and is relatively very small and quite approximate. The values of k2 can be used to obtain electronic and steric profiles from which it can be concluded that bond-making is very low for reactions of the intrinsically much more reactive Rh4(CO)10(PCy3)2, but significant for reactions of the intrinsically less reactive Rh4(CO)9(etpb)3. Steric effects due to different nucleophile cone angles are consequently much less pronounced for Rh4(CO)10(PCy3)2 than for Rh4(CO)9(etpb)3. Comparison with data reported elsewhere for Rh4(CO)9{HC(PPh2)3} shows that the intrinsic susceptibility of these clusters towards nucleophilic attack increases along the series Rh4(CO)9{HC(PPh2)3} < Rh4(CO)9(etpb)3 < Rh4(CO)10(PCy3)2 whereas the degree of bond-making, and the consequent steric effects, decrease along this series. 31P(1H) NMR studies of Rh4(CO)9(etpb)3 suggest that it exists as two fluxional isomers but this does not affect the discussion of the kinetics. The associative reactions of Rh4(CO)10(PCy3)2 lead mainly to fragmentation whereas those of Rh4(CO)9(etpb)3 lead mainly to substitution. © 1990.