The optimized structure of complex 2 (Figure , b) confirms its assignment as a genuine H2 complex with an H-H bond length is 0.836 Å, in good agreement with the experimental estimation (0.86 Å). The C=C backbone is slightly elongated (1.427 Å) as consequence of πbackdonation from the nickel center and exhibits a sum of valence angles of 354.4º and 351.6º around C10 and C41, respectively. Interestingly, the H-H vector is out of the Ni-olefin plane, suggesting that both ligands are receiving π-backdonation from different d-orbitals. Wellcharacterized Ni(0)-H2 complexes were hitherto limited to those incorporating an additional sligand or a s-acceptor, Z-type ligand. Such ligands can assist coordination by lowering the energy of the s-antibonding d-orbital and, in some cases, allow H2 deprotonation to form strongly hydridic d 10 hydrides. The observation of complex 2 demonstrates that a single p-acidic olefin ligand is sufficient to stabilize a H2 complex of a d 10 metal and can cooperatively generate an active hydride without the need for full oxidative addition to a dihydride intermediate, warranting consideration of such pathways in catalytic hydrogenation reactions.