Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).

Reaction of [LtBuNiBr] (LtBu = [HC(C(CMe3)NC6H3(iPr)2)2]−) with KC8 in toluene solution yields the complex [LtBuNi(toluene)], 1, where toluene is bound in a η2 mode via a CC unit of the aromatic ring, as revealed by single-crystal X-ray crystallography and DFT calculations (B3LYP/6-31G*). Performing the same reaction in hexane as the solvent did not lead to a traceable product, so that the β-diketiminato ligand system was changed from LtBu to the less bulky LMe (LMe = [HC(CMeNC6H3(iPr)2)2]−). Reduction of [LMeNiBr]2 with KC8 in diethyl ether led to [LMeNi]2, 2, with intramolecular Ni−aryl interactions, while employment of [LMeNi(μ-Br)2Li(thf)2] as a precursor for a reaction with KC8 in OEt2 led to the complex [LMeNi(μ-Br)Li(thf)2]2, 3. Both complexes 2 and 3 could be fully characterized, also with the aid of XRD, and their reactivity with respect to H2 and N2 was examined. It turned out that they oxidatively add H2 to give the known compound [LMeNi(μ-H)]2, I, while the reaction with N2 provides the dinitrogen complex [(LMeNi)2(N2)], 4.

After single electron reduction of the dinitrogen complex [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (I) with KC 8 , reaction of the resulting compound K[L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (II) with sodium sand yields KNa[L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (1), which contains two different alkali metal ions. Treatment of I with two equivalents of sodium sand leads to the symmetric complex Na 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (2). Complexes 1 and 2 were investigated by single crystal X-ray diffraction analysis as well as by Raman spectroscopy, and the results were compared with the data of K 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] (III), which 1169 contains two K + ions. Thus, it became obvious that the nature of the alkali metal ion M in compounds M 2 [L tBu Ni(μ-η 1 :η 1 -N 2 )NiL tBu ] has hardly any influence on the degree of NN bond activation. Furthermore, it was shown that treatment of the dinickel(I) complex III with CO leads to the dinickel(0) compound K 2 [L tBu Ni(CO)] 2 (4) and N 2 . Reaction of the unreduced dinickel(I) complex I with CO leads to a more simple replacement of the N 2 ligand and formation of [L tBu Ni(CO)] (3). Scheme 1.Stepwise reduction of I. [1]

The preparation of a novel dinuclear nickel(ii) hydride complex and its reactivity that often leads to nickel(i) compounds is described.

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)(2)NiL], 1 (L = [HC(CMeNC(6)H(3)(iPr)(2))(2)](-)), has been carried out. While the reaction with one equivalent of potassium graphite, KC(8), led to the mixed valent Ni(I)/Ni(II) complex K[LNi(μ-H)(2)NiL], 3, treatment of 1 with two equivalents of KC(8) surprisingly yielded in the trinuclear complex K(2)[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 4, in good yields. The Ni(3)H(4) core contains one Ni(II) and two Ni(I) centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni(II)(2)Ni(I) compound K[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 5, and treatment with CO leads to the elimination of H(2) with formation of the carbonyl complex K(2)[LNi(CO)](2), 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D(4) reacts with H(2) to give back 4. The crystal structures of the novel compounds 3-6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.