Reported N2 complexes of cobalt do not have substantial weakening of the N−N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N−N bond, but in formally zerovalent complexes, cobalt and iron give similar N−N weakening. The weakening is due to cobalt-to-N2 π-backbonding, and potassium cations pull more electron density into N2. These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N−N bond of N2.

We report the formation and N 2 reactivity of novel cobalt hydride complexes supported by bulky β-diketiminate ligands (L). Addition of KHBEt 3 to LCoCl gives [LCo(μ-H)] 2 (1) or K 2 [LCoH] 2 (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N 2 at room temperature to give dinuclear N 2 complexes with loss of H 2 .Complexes of a transition metal (defined here as a metal with a partially filled d shell) having only three bonds to the metal are interesting because the orbital energies, spin states, and reaction pathways can be different than traditional complexes. 1 Isolating three-coordinate complexes invariably depends on the use of extremely bulky supporting ligands, which protect the metal center through steric effects. However, this strategy might seem to be incompatible with three-coordinate hydride complexes, because H -is the smallest possible ligand. Accordingly, no three-coordinate hydride complexes are known. 2 New kinds of hydride complexes are desired because of the many roles for hydrides in organometallic chemistry and catalysis. 3 Using iron complexes, we recently introduced the use of the very bulky bidentate β-diketiminate ligand L (L = 2,2,6,6-tetramethyl-3,5-bis(2,4-diisopropylphenylimido)hept-4-yl) for enabling the isolation of three-coordinate complexes in which one of the three ligands is small (e.g. halide, CH 3 ). 4 Here, we report that L can be used to stabilize the first crystallographically characterized three-coordinate hydride complex of any transition metal. We also report that this cobalt-hydride complex and another related complex react with N 2 at room temperature and atmospheric pressure, through the bimetallic reductive elimination of H 2 . Figure 1 shows the syntheses and structures of dimeric cobalt(II) hydride (1) and cobalt(I) hydride (2) complexes, which come from adding different amounts of KHBEt 3 to toluene solutions of the three-coordinate cobalt(II) complex LCoCl under Ar. 5 Addition of 1 equiv of KHBEt 3 to LCoCl gives [LCo(μ-H)] 2 (1) in 72% yield. Although we were not able to completely free 1 from impurities (see Supporting Information), several forms of characterization have been possible. The X-ray crystal structure of 1 reveals that it has two hydride ligands (located in the difference Fourier map) that bridge diketiminate-bound cobalt centers. The metal coordination and geometry closely resemble those for the iron(II) complex [LFe(μ-H)] 2 6 and a dimeric β-diketiminate nickel(II) hydride complex recently reported by Limberg. 7 The cobalt atoms in 1 are separated by 2.476 (5) the metal-metal distances in the iron (2.624(2) Å) and nickel (2.3939(6) Å) analogues. The presence of the hydride ligands in 1 was confirmed through the reaction of a toluene solution of 1 with 2 equiv of cyclohexene to give the cobalt(II) cyclohexyl product LCo(C 6 H 11 ), which results from [1,2]-addition of the cobalt h...