Hydride Reactivity of Ni II XNi II Entities: Mixed‐Valent Hydrido Complexes and Reversible Metal Reduction

After the lithiation of PYR‐H 2 (PYR 2− =[{NC(Me)C(H)C(Me)NC 6 H 3 ( i Pr) 2 } 2 (C 5 H 3 N)] 2− ), which is the precursor of an expanded β‐diketiminato ligand system with two binding pockets, its reaction with [NiBr 2 (dme)] led to a dinuclear nickel(II)–bromide complex, [(PYR)Ni(μ‐Br)NiBr] ( 1 ). The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(μ‐SEt)NiBr] ( 3 ). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt 3 . This treatment afforded [(PYR)Ni(μ‐H)Ni] ( 2 ), which is a mixed valent Ni I μ‐HNi II complex, and [(PYR‐H)Ni(μ‐SEt)Ni] ( 4 ), in which two tricoordinated Ni I moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the Ni I centres. The addition of a mild acid to complex 4 leads to the elimination of H 2 and the formation of a Ni II Ni II compound, [(PYR)Ni(μ‐SEt)NiOTf] ( 5 ), so that the original Ni II (μ‐SEt)Ni II X core of compound 3 is restored. All of these compounds were fully characterized, including by X‐ray diffraction, and their molecular structures, as well as their formation processes, are discussed.