Ab Initio Calculations of pKaValues of Transition-Metal Hydrides in Acetonitrile

A well-defined ab initio protocol was developed which could be used to calculate the pKavalues of a variety of transition-metal hydrides in acetonitrile with a precision of 1.5 pKaunits. The gas-phase energies were calculated using a two-layer ONIOM method where the core layer was handled by the CCSD(T) theory. In addition, the C-PCM solvation model previously developed for small organic molecules was confirmed to be reliable for predicting the solvation energies of bulky organometallic complexes. Using the protocol, we studied the structure-property relationships for the acidities of the Fe-, Co-, and Ni-group metal hydrides. It was found that the pKavalues of these hydrides varied considerably with respect to the metal centers. The third-row metal hydrides always appeared to be the least acidic, as compared to the first- and second-row metal hydrides. On the other hand, the relative acidity between the first- and second-row metal hydrides was strongly dependent on the chemical formulas of the metal complexes. Finally, linear correlations were observed between the pKavalues of metal hydrides and the NBO (natural bond orbital) charges carried by the chelating phosphorus atoms.