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

A simple equation (pKaTHF = ∑AL + Ccharge + Cnd + Cd6) can be used to obtain an estimate
of the pKa of diamagnetic transition metal
hydride and
dihydrogen complexes in tetrahydrofuran, and, by use of conversion
equations, in other solvents. It involves adding acidity constants AL 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 (Ccharge = −15, 0 or 30 for x =
+1, 0 or −1 charge, respectively) and the periodic row of the
transition metal (Cnd = 0 for 3d or 4d
metal, 2 for 5d metal) as well as a correction for d6 octahedral
acids (Cd6 = 6 for d6 metal
ion in the acid, 0 for others) that are not dihydrogen complexes.
Constants AL are provided for 13 commonly
occurring ligand types; of these, nine neutral ligands are correlated
with Lever’s electrochemical ligand parameters EL. This method gives good estimates of the over 170 literature
pKa values that range from less than zero
to 50 with a standard deviation of 3 pKa 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 pKa 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.