Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

Understanding the
thermodynamics of paramagnetic transition metal
hydride complexes, especially of the abundant 3d metals,
is important in the design of electrocatalysts and organometallic
catalysts. The pKaMeCN([MHLn]+/[MLn) of paramagnetic hydrides in
MeCN are estimated for the first time using the ligand acidity constant
(LAC) equation where contributions to the pKaMeCN from each ligand are simply added together,
with the sum corrected for effects of charge and 5d metals. The pKaLAC–MeCN([MHLn]+/MLn) of over 200 hydride
complexes MHLn are used, along with their electrochemical
potentials from the literature, in an uncommonly applied thermochemical
cycle in order to reveal systematic trends in the redox couples MIII/II and MV/IV (M = Cr, Mo, W), MnII/I, ReVI/V and ReIV/III, MIII/II and
MIV/III (M = Fe, Ru, Os), and MIII/II and MII/I (M = Co, Rh, and Ir) and allow the estimation of the bond
dissociation free energies BDFE­(MH) of the unoxidized hydrides MHLn and the prediction of the electrochemical potential for their
oxidation. Density functional theory (DFT) calculations are used to
validate the pKaLAC–MeCN values of hydrides of WIII, MnII, FeIII, RuIII, CoII, and NiIII. When a
pKaLAC–MeCN is less
than zero for a given complex [MHLn]+, the oxidation
of MHLn is irreversible due to proton loss from the oxidized
complex to the solvent. When pKaLAC–MeCN ≫ 0, the oxidation is reversible when there is no gross change
in the coordination geometry upon a change in the redox state. Twenty
paramagnetic hydrides prepared in bulk all have pKaLAC–MeCN > 8.