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Controlling the heterolytic cleavage of the H-H bond of dihydrogen is critically important in catalytic hydrogenations and in the catalytic oxidation of H 2 . We show how the rate of reversible heterolytic cleavage of H 2 can be controlled, spanning 4 orders of magnitude at 25 °C, from 2.1 × 10 3 s -1 o ≥10 7 s -1 . Bifunctional Mo complexes, [CpMo(CO)(κ 3 -P 2 N 2 )] + (P 2 N 2 = 1,5-diaza-3,7-diphosphacyclooctane diphosphine ligand with alkyl/aryl groups on N and P), have been developed for heterolytic cleavage of H 2 into a proton and a hydride, akin to frustrated Lewis pairs. The H-H bond cleavage is enabled by the basic amine in the second coordination sphere. The products of heterolytic cleavage of H 2 , Mo hydride complexes bearing protonated amines, [CpMo(H)(CO)(P 2 N 2 H)] + , were characterized by spectroscopic studies and by X-ray crystallography. Variable-temperature 1 H, 15 N, and 2-D 1 H- 1 H ROESY NMR spectra indicated rapid exchange of the proton and hydride. The exchange rates are in the order [CpMo(H)(CO)(P Ph  2 N Ph  2 H)] + &gt; [CpMo(H)(CO)(P   Bu  2 N Ph  2 H)] + &gt; [CpMo(H)(CO)(P Ph  2 N Bn  2 H)] + &gt; [CpMo(H)(CO)(P   Bu  2 N Bn  2 H)] + &gt; [CpMo(H)(CO)(P   Bu  2 N   Bu  2 H)] + . The pK a values determined in acetonitrile range from 9.3 to 17.7 and show a linear correlation with the logarithm of the exchange rates. This correlation likely results from the exchange process involving key intermediates that differ by an intramolecular proton transfer. Specifically, the proton-hydride exchange appears to occur by formation of a molybdenum dihydride or dihydrogen complex, resulting from proton transfer from the pendant amine to the metal hydride. The exchange dynamics are controlled by the relative acidity of the [CpMo(H)(CO)(P 2 N 2 H)] + and [CpMo(H 2 )(CO)(P 2 N 2 )] + isomers, providing a design principle for controlling heterolytic cleavage of H 2 .

Reversible Heterolytic Cleavage of the H–H Bond by Molybdenum Complexes: Controlling the Dynamics of Exchange Between Proton and Hydride