Four new rhodium complexes, [Rh(depx) 2](CF 3SO 3), [(H) 2Rh(depx) 2](CF 3SO 3), [HRh(depx) 2(CH 3CN)](CF 3SO 3) 2, and HRh(depx) 2 (where depx = α,α′-bis(diethylphosphino)xylene), have been synthesized and characterized. The pK a values of [(H) 2Rh(depx) 2] + (30.6 ± 0.3) and [HRh(depx) 2(CH 3CN)] 2+ (11.5 ± 0.4) in acetonitrile were determined by equilibrium reactions with appropriate bases. The hydride donor ability of [(H) 2Rh(depx) 2] + (71.6 ± 1.0 kcal/mol) was determined by the heterolytic cleavage of hydrogen by [HRh(depx) 2(CH 3CN)] 2+ to form [(H) 2Rh(depx) 2] + in the presence of thioacetamide or benzamide. These equilibrium measurements, the half-wave potentials for the Rh(+l/0) and Kh(0/-l) couples of [Rh(depx) 2] +, and the reversible one-electron oxidation of HRh(depx) 2 (all measured at 22 ± 1.5 °C) 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry