Abstract. In the context of biological nitrogen fixation, the question whether molybdenum or iron is the site for substrate binding and reduction is still discussed controversially. Therefore, the development of relevant model compounds containing early or late transition metals is essential in understanding the nature and behavior of intermediates bound to the FeMo-cofactor. Ligated early transition metal atoms are known to strongly activate N 2 and mediate its cleavage, but the resulting complexes contain metal-nitrogen bonds, which often elude

The reaction of [L(tBu)Ni(OEt(2))] (where L(tBu) = [HC(C(t)BuNC(6)H(3)((i)Pr)(2))(2)](-)) with phenylhydrazine leads to the phenylhydrazido(1-) complex [L(tBu)Ni(η(2)-NPhNH(2))] (1) with concomitant formation of H(2). Treatment of 1 with potassium graphite in the presence of crown ether again leads to H(2) evolution and affords the heterobimetallic complex [L(tBu)Ni(μ-η(2):η(2)-NPhNH)]K(18-crown-6) containing the doubly deprotonated phenylhydrazido(2-) ligand. 1 can be converted into a phenyldiazenido complex [L(tBu)Ni(η(1)-NNPh)] in the course of a dehydrogenation reaction employing 1,2-diisopropylazo dicarboxylate (DIAD) as the oxidant.

A (β‐diketiminato)nickel(II) hydrazido(1–) complex [LtBuNi(η2‐N2H3)], {1, LtBu = [HC(CtBuNC6H3{iPr}2)2]–} has been obtained by treatment of [LtBuNiBr] with hydrazine. In a reaction of 1 with two equivalents of the azo compound diisopropyl azodicarboxylic ester (adc–OiPr) the Ni(N2H3) entity acts as both a hydrogenating and a reducing agent: diisopropyl hydrazidodicarboxylate (hdc–OiPr) is formed, and more adc–OiPr is reduced by two electrons. The resulting (adc–OiPr)2– is found as a ligand in the ultimate nickel product complex, the trinuclear nickel(II) compound [LtBuNi(μ‐adc–OiPr)Ni(μ‐adc–OiPr)NiLtBu] (2), in which two LtBuNi+ units are linked by a [NiII(adc–OiPr)2]2– moiety. The hypothesis that LtBuNiI species are acting as intermediates was supported by the independent finding that 2 can also be obtained by reaction of [LtBuNi(OEt2)] with adc–OiPr.

The β-diketiminato nickel(II) hydrazido(-1) complex [L tBu Ni(η 2 -N 2 H 3 )], (IV) was investigated with respect to its deprotonation/protonation behavior. Deprotonation with KOtBu led to [L tBu Ni(μ,η 2 :η 2 -N 2 H 2 )K(solv)] (1), and this process was proven to be reversible. Spectroscopic and DFT studies revealed an electronic structure intermediate between nickel(II) hydrazido(-2) and nickel (0) diazene. On the [a]