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

The hydride complex [Pt(dmpe)(2)H](+) (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 DeltaGdegrees 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 ability, DeltaGdegrees(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/O) values for five rhenium carbonyl complexes were measured by cyclic voltammetry. Combined with the known DeltaGdegrees(H-) values for the complexes, the hydrogen atom donating abilities could be determined. These values were all found to be approximately 50 kcal/mol.