Quantification of Thermodynamic Hydridicity of Hydride Complexes of Mn, Re, Mo, and W Using the Molecular Electrostatic Potential.

The molecular electrostatic potential (MESP) at the hydride nucleus, V, is proposed as a powerful measure of the hydride donor ability (hydridicity) of metal hydride complexes. V values have been determined for several group VI and group VII octahedral metal hydride complexes of Mo, W, Mn, and Re at the B3LYP level of DFT. Further, the hydridicity, defined by the thermodynamic parameter ΔG° is determined for all of these complexes using a thermodynamic cycle that describes hydride abstraction reactions by HO ions. The ΔG° of most of the W and Mo complexes corresponding to the reaction with HO are lower than 20 kcal/mol, whereas a majority of other complexes showed ΔG° in the range of 20-60 kcal/mol. In all cases, a lower value of ΔG° is correlated to a higher negative V value. The increase in the negative character of V indicated higher hydridicity of the complex and easy elimination of the hydride ion. Thus, the MESP approach provided a simple yet accurate prediction of the hydride donor ability of the metal hydride complex compared to a more tedious and demanding calculation to obtain the thermodynamic parameter. This approach and its applicability are validated by correlating V with experimentally known ΔG° values of W and Mo hydride complexes.