Metallacarboranes in Catalysis. 5. Interconversion of closo-Bis(phosphine)hydridorhodacarboranes by Rhodium Transfer between η5-nido-Carborane Anions

Article abstract of DOI:10.1021/ja00322a041

Both the hydride and dicarbollide ligands of a series of closo, formally six-coordinate Rh(III), bis(triphenylphosphine)hydrido η⁵-(nido-C2B9H9RR') complexes were easily replaced by a series of (1-) ligands using thermal reactions.The following trend for ease of displacement of the nido-carborane anion in cage-carbon-substituted complexes has been found: 7,8-disubstituted > 7,8-monosubstituted > 7,8-unsubstituted > 7,9-unsubstituted ca. 2,9-unsubstituted.Kinetic studies of the reaction of (R = Me, R' = H) (IVd) with (1-) (Ia) and (1-) (III) in THF at 29 deg C showed no anion concentration dependence and a common first-order rate constant for the two reactions although in the case of (1-) (II), anion dependence was observed.Similar kinetic studies of the reaction of (IVf) with anions Ia, II, and III in THF at -63 deg C proved these exchanges with exo-nido substrates to be much faster than the reactions involving closo substrates.In the cases of Ia and II, the less stable kinetic product of the reaction, the exo-nido tautomer, was initially observed along with its conversion to the more stable closo tautomer.Analysis of the anion concentration dependence of the rates of these reactions suggested the existence of a spectroscopically invisible intermediate, presumably an isomer of the exo-nido starting complex.Formation of an analogous intermediate appears to be the rate-determining step in reaction of closo-IVd with anions Ia and III.In addition, the dissociation of either PPh3 or nido-carborane anion does not appear to be involved in any of these cage exchange reactions.Anion III was found to be a more effective nucleophile than Ia which, in turn, was more nucleophilic than II.