Hydride donor abilities and bond dissociation free energies of transition metal formyl complexes

The hydride complex [Pt(dmpe)₂H]⁺ (dmpe = 1,2-bis(dimethylphosphino)ethane) reversibly transfers H^- to the rhenium carbonyl complex [CpRe(PMe₃)(NO)(CO)]⁺, giving the formyl CpRe(PMe₃)(NO) (CHO). From the equilibrium constant for the hydride transfer (16.2), the ΔG° for the reaction was determined (-1.6 kcal/mol), as was the hydride-donating ability of the formyl (44.1 kcal/mol). The hydride-donating donating ability, ΔG°_H-, is defined as the energy required to release the hydride ion into solution by the formyl complex [i.e. M(CHO) → M(CO)⁺ + H^-]. Subsequently, the hydride-donating ability of a series of formyl complexes was determined, ranging from 44 to 55 kcal/mol. With use of this information, two rhenium carbonyl complexes, [CpRe(NO)(CO)₂]⁺and [Cp*Re(NO)(CO)₂]⁺, were hydrogenated to formyls, employing [Pt(dmpp)₂]²⁺ and Proton-Sponge. Finally, the E_1/2(I/0) values for five rhenium carbonyl complexes were measured by cyclic voltammetry. Combined with the known ΔG°_H- values for the complexes, the hydrogen atom donating abilities could be determined. These values were all found to be approximately 50 kcal/mol.