Estimating the Acidity of Transition Metal Hydride and Dihydrogen Complexes by Adding Ligand Acidity Constants

A simple equation (pK_a^THF = ∑A_L + C_charge + C_nd + C_d6) can be used to obtain an estimate of the pK_a of diamagnetic transition metal hydride and dihydrogen complexes in tetrahydrofuran, and, by use of conversion equations, in other solvents. It involves adding acidity constants A_L for each of the ligands in the 5-, 6-, 7-, or 8-coordinate conjugate base complex of the hydride or dihydrogen complex along with a correction for the charge (C_charge = −15, 0 or 30 for x = +1, 0 or −1 charge, respectively) and the periodic row of the transition metal (C_nd = 0 for 3d or 4d metal, 2 for 5d metal) as well as a correction for d⁶ octahedral acids (C_d6 = 6 for d⁶ metal ion in the acid, 0 for others) that are not dihydrogen complexes. Constants A_L are provided for 13 commonly occurring ligand types; of these, nine neutral ligands are correlated with Lever’s electrochemical ligand parameters E_L. This method gives good estimates of the over 170 literature pK_a values that range from less than zero to 50 with a standard deviation of 3 pK_a units for complexes of the metals chromium to nickel, molybdenum, ruthenium to palladium, and tungsten to platinum in the periodic table. This approach allows a quick assessment of the acidity of hydride complexes found in nature (e.g., hydrogenases) and in industry (e.g., catalysis and hydrogen energy applications). The pK_a values calculated for acids that have bulky or large bite angle chelating ligands deviate the most from this correlation. The method also provides an estimate of the base strength of the deprotonated form of the complex.