Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(I) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies

Thermodynamic and kinetic parameters for the oxidative addition of H2 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.