Thermodynamic studies of HRh(depx)2 and [(H)2Rh(depx)2](CF3SO3):: Relationships between five-coordinate monohydrides and six-coordinate dihydrides

Four new rhodium complexes, [Rh(depx)(2)](CF3SO3), [(H)(2)Rh(depx)(2)](CF3SO3),[HRh(dePX)(2)(CH3CN)](CF3SO3)(2), and HRh(depx)(2) (where depx = alpha,alpha'-bis(diethylphosphino)xylene), have been synthesized and characterized. The pK(a) values of [(H)(2)Rh(depx)(2)](+) (30.6 +/- 0.3) and [HRh(depx)(2)(CH3CN)](2+) (11.5 +/- 0.4) in acetonitrile were determined by equilibrium reactions with appropriate bases. The hydride donor ability of [(H)(2)Rh(depx)(2)](+) (71.6 +/- 1.0 kcal/mol) was determined by the heterolytic cleavage of hydrogen by [HRh(depx)(2)(CH3CN)](2+) to form [(H)(2)Rh(depx)(2)](+) in the presence of thioacetamide or benzamide. These equilibrium measurements, the half-wave potentials for the Rh(+1/0) and Rh(0/-1) couples of [Rh(depx)(2)](+), and the reversible one-electron oxidation of HRh(depx)(2) (all measured at 22 +/- 1.5 degreesC) were used to establish nine heterolytic and homolytic bond-dissociation free energies in acetonitrile. From these relationships, and those established previously for analogous cobalt and platinum complexes, we conclude that the homolytic bond-dissociation free energies of six-coordinate dihydride species are approximately 2 kcal/mol less than those of the corresponding five-coordinate monohydride species derived by deprotonation. Similarly, the pK(a) values of six-coordinate dihydride complexes are approximately 2 units lower than the corresponding five-coordinate species formally derived by homolytic bond cleavage.