Ab Initio Calculations of Thermodynamic Hydricities of Transition-Metal Hydrides in Acetonitrile

Abstract

Theoretical hydricities calculated by different methods are compared with all the available experimental values for the Co- and Ni-group metal hydrides in acetonitrile. It is found that the B3P86 method employing a sufficiently flexible basis set in combination with the C-PCM solvent model can accurately predict the hydricity with a precision of 2.0 kcal/mol. The same method can also accurately predict the acidities of the metal−hydrogen bond and the redox potentials of the corresponding metal hydrides with precisions of 1.9 pKa units and 0.07 V, respectively. On the other hand, an ONIOM method where the core layer is treated with the CCSD(T) method fails to predict the hydricity of transition-metal hydrides, possibly because the electronic effects of the outer layer are not accurately estimated with the low-level theory in the ONIOM partitioning scheme or because the basis sets in the ONIOM methods are not sufficiently flexible. As to the periodical trends, it is found that for the Co-group metal hydrides, the hydricities increase in the order second row < third row < first row. For the Ni-group metal hydrides, the hydricities increase in the order second row ≈ third row ≪ first row. As to the effects of the phosphine ligands, it is found that a dual-parameter equation can quantitatively predict the hydricities, where the two parameters are the NBO charge of the phosphorus and the natural bite angle of the diphosphine. Finally, no correlation is found between the hydricity and the metal−hydrogen vibration frequency or the hydrogen chemical shift. Therefore, it is not reliable to use any vibrational spectrum or proton NMR method to estimate the hydricity of transition-metal hydrides.