Binuclear complexes of nickel(0): Comparison of a bridging methyl isocyanide and a bridging (methylamino)carbyne ligand

Article abstract of DOI:10.1021/om00140a008

The reduction of the Ni^I species [Ni₂(μ-CNMe)(CNMe)₃(PPh₂CH ₂PPh₂)₂][PF₆]₂ (1) with sodium amalgam leads to the formation of the complex Ni₂⁰(μ-CNMe)(CNMe)₂(PPh₂CH ₂PPh₂)₂ (2), the crystal and molecular structure of which has been determined. Complex 2 crystallizes in the monoclinic space group C2/c with a = 22.645 (3) A?, b = 12.662 (2) A?, c = 35.126 (4) A?, β = 98.017 (9)°, V = 9973.0 (4) A?³, and Z = 8. The structure was refined to convergence leading to R and R_w of 0.060 and 0.082, respectively, for 3682 observations in the range 6° ≤ 2θ ≤ 110° with I ≥ 3.0σ(I) (Cu Kα radiation). The Ni-Ni separation is 2.572 (1) A? consistent with a Ni-Ni bond. The molecular structure of 2 displays unusual cis,cis bridging diphosphine ligands. The bridging CNMe ligand of the complex is very basic and can be easily protonated by weak acids to form [Ni₂⁰(μ-CNMeH)(CNMe)₂(PPh₂CH ₂PPh₂)₂]·[PF₆] (3), the crystal and molecular structure of which also has been determined. Complex 3 crystallizes in the monoclinic space group Cc with a = 23.124 (3) A?, b = 12.941 (2) A?, c = 20.792 (3) A?, β = 117.00 (1)°, V = 5544.0 (3) A?³, and Z = 4. The structure was refined to convergence leading to R and R_w of 0.057 and 0.073, respectively, for 3435 observations in the range 6° ≤ 2θ ≤ 120° with I ≥ 3.0σ(I). In comparison to complex 2, 3 possesses a shorter Ni-Ni separation of 2.500 (1) A?, shorter C-Ni bond distances for the bridging ligand, and longer Ni-C bond lengths for the terminal CNMe groups. These observations, along with spectroscopic data, are interpreted in terms of a greater contribution of an aminocarbyne valence bond description to the bonding of 3 compared to 2.