Cooperative H 2 activation at a nickel(0)-olefin centre.

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H 2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H 2 ) intermediate. Here we provide evidence for an alternative H 2 -activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H 2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H 2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H 2 . The usefulness of this cooperative H 2 -activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H 2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.