Brønsted–Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

Transition
metal hydride complexes are usually amphoteric, not
only acting as hydride donors, but also as Brønsted–Lowry
acids. A simple additive ligand acidity constant equation (LAC for
short) allows the estimation of the acid dissociation constant KaLAC of diamagnetic transition metal
hydride and dihydrogen complexes. It is remarkably successful in systematizing
diverse reports of over 450 reactions of acids with metal complexes
and bases with metal hydrides and dihydrogen complexes, including
catalytic cycles where these reactions are proposed or observed. There
are links between pKaLAC and
pKaTHF, pKaDCM, pKaMeCN for neutral and cationic acids. For the groups from chromium to
nickel, tables are provided that order the acidity of metal hydride
and dihydrogen complexes from most acidic (pKaLAC −18) to least acidic (pKaLAC 50). Figures are constructed showing metal
acids above the solvent pKa scales and
organic acids below to summarize a large amount of information. Acid–base
features are analyzed for catalysts from chromium to gold for ionic
hydrogenations, bifunctional catalysts for hydrogen oxidation and
evolution electrocatalysis, H/D exchange, olefin hydrogenation and
isomerization, hydrogenation of ketones, aldehydes, imines, and carbon
dioxide, hydrogenases and their model complexes, and palladium catalysts
with hydride intermediates.