Thermodynamic and kinetic parameters for the oxidative addition of H 2 to [RhI(bpy)2]+ (bpy = 2,2′-bipyridine) to form [RhIII(H)2(bpy) 2]+ were determined from either the UV-vis spectrum of equilibrium mixtures of [RhI(bpy)2]+ and [RhIII(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [RhIII(H)2(bpy)2] + as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol-1 and -18 cal mol-1 K-1, respectively. The reaction enthalpy and entropy are -10.3 kcal mol-1 and -19 cal mol-1 K-1, respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm3 mol-1 in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm3 mol-1 in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Å, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Å, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.
ASJC Scopus subject areas
- Inorganic Chemistry