Ligand Protonation Triggers H(2) Release from a Dinickel Dihydride Complex to Give a Doubly “T”‐Shaped Dinickel(I) Metallodiradical

The dinickel(II) dihydride complex (1(K)) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L(−)), providing two pincer‐type {N(3)} binding pockets, has been reported to readily eliminate H(2) and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2(Na), 2(K)) of L(−) are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local [Formula: see text] type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e(−) substrate reductions as shown by the rapid reaction with H(2) or O(2). X‐ray crystallography reveals distinctly different positions of the K(+) in 1(K) and 2(K), suggesting a stabilizing interaction of K(+) with the dihydride unit in 1(K). H(2) release from 1(K) is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H(2) elimination from the bimetallic cleft.