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 H₂ to a metal centre or by deprotonation of a non-classical metal dihydrogen (M–H₂) intermediate. Here we provide evidence for an alternative H₂-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H₂ 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–H₂ complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H₂. The usefulness of this cooperative H₂-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H₂ activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)–(Z) isomerization and catalyst degradation by self-hydrogenation.

催化烯烃加氢在有机合成中普遍存在。在大多数提出的均相催化循环中，反应性 M-H 键是通过 H ₂氧化加成到金属中心或通过非经典金属二氢 (M-H ₂ ) 中间体的去质子化产生的。在这里，我们提供了另一种H ₂活化机制的证据，该机制涉及从金属结合的H ₂分子到金属配位烯烃的直接配体到配体氢转移（LLHT）。一种不寻常的钳式配体，具有两个膦配体和一个中心烯烃，支持非经典 Ni-H ₂配合物和 LLHT 的 Ni(烷基)(氢化) 产物的形成，与溶解的 H ₂快速平衡。这种协同H ₂活化机制对于催化的有用性在二苯乙炔的半氢化中得到了证明。实验和计算机理研究支持LLHT 在H ₂活化和催化半氢化中的核心作用。获得的产物分布很大程度上取决于( E )-(Z)异构化和催化剂自加氢降解之间的竞争。