Ab initio calculations of pKa values of transition-metal hydrides in acetonitrile

A well-defined ab initio protocol was developed which could be used to calculate the pK(a) values of a variety of transition-metal hydrides in acetonitrile with a precision of 1.5 pK(a) units. 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 pK(a) values 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 pK(a) values of metal hydrides and the NBO ( natural bond orbital) charges carried by the chelating phosphorus atoms.