A compartmental ligand scaffold HL with two beta-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi2(H)2], M = Na (Na center dot 2) and K (K center dot 2), which were isolated after reacting the precursor complex [LNi2(mu-Br)] (1) with MHBEt3 (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K center dot 2 with dibenzo(18-crown-6) led to the separated ion pair [LNi2(H)(2)]-[K(DB18C6)] (2[K(DB18C6)]). Reaction of Na center dot 2 or K center dot 2 with D-2 was investigated by a suite of H-1 and H-2 NMR experiments, revealing an unusual pairwise H-2/D-2 exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of similar to 62 kJ/mol, to give H-2 and an antiferromagnetically coupled [LNi2I](-) species. This was confirmed by SQUID monitoring during H-2 release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na+ or K+) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)(2)] -> H-2 transition derived from 2D H-1 EXSY spectra confirmed first-order dependence of H-2 release on M center dot 2 concentration and a strong effect of the alkali metal cation M. Treating [LNi2(D)(2)](-) with phenylacetylene led to D-2 and dinickel(II) complex 3(-) with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi2II(H)(2)](-) having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H-2 upon substrate binding is reminiscent of the proposed N-2 binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.