Brønsted–Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

• Published in: Chemical reviews Vol. 116; no. 15; pp. 8588 - 8654
• Main Author: Morris, Robert H
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10.08.2016
• Subjects: Acidity; Catalysts; Hydrogen; Hydrogenation; Oxidation
• ISSN: 0009-2665; 1520-6890
• DOI: 10.1021/acs.chemrev.5b00695

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 K a LAC 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 pK a LAC and pK a THF, pK a DCM, pK a MeCN 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 (pK a LAC −18) to least acidic (pK a LAC 50). Figures are constructed showing metal acids above the solvent pK a 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.