Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H ₂ have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P ₂ ^Ph N ₂ ^Ph ) ₂ (CH ₃ CN)](BF ₄ ) ₂ , 3a (where P ₂ ^Ph N ₂ ^Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H ₂ production rate to be determined. The turnover frequency of 350 s ⁻¹ establishes that the rate of H ₂ production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P ₂ ^Cy N ₂ ^Bz ) ₂ ](BF ₄ ) ₂ , 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b ( K _eq = 190 atm ⁻¹ at 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A–4C, which have been identified by a combination of one- and two-dimensional ¹ H, ³¹ P, and ¹⁵ N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P ₂ ^Cy N ₂ ^Bz ) ₂ ](BF ₄ ), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.

The cobalt analogue of a highly active nickel electrocatalyst for hydrogen production has been synthesized and characterized as [Co(P^Ph ₂N^Ph ₂)₂(CH₃CN)](BF₄)₂. In the presence of triflic acid in acetonitrile solution, the complex loses one cyclic diphosphine ligand to form [Co(P^Ph ₂N^Ph ₂)(CH₃CN)₃](BF₄)₂, which has been synthesized independently and stucturally characterized. The latter complex serves as an electrocatalyst for hydrogen formation with a turnover frequency of 90 s⁻¹ and an overpotential of 285 mV using bromoanilinium tetrafluoroborate as the acid. A similar cobalt complex with a related diphosphine ligand that does not contain a pendant base is not catalytically active, confirming an important role for the pendant amine in the catalytic reaction.