Crystal and Molecular Structure of [Ni(Me₄[14]hexaenatoN₄)]₂(CF₃SO₃)₂: Conversion of a Metal-Stabilized π-Radical to a Diamagnetic Dimer with a Long Metal-Metal Bond and Eclipsed Macrocyclic Ligands upon Crystallization

In dilute nonaqueous solutions the tetraaza macrocyclic complex [Ni(Me₄14]hexaenatoN₄)]⁺ exists as a monomeric paramagnetic species, whose spectrum has previously been analyzed in terms of a Ni(II)-stabilized 15-π electron cation radical with the unpaired electron delocalized over the 14-membered ring system. However, as shown by the present three-dimensional x-ray diffraction study, the cation in the crystalline state forms a diamagnetic dimer. The trifluoromethanesulfonate salt crystallizes with one dipositive dimer per unit cell in the triclinic space group [Formula omitted] with a = 12.069 (2) Å, b = 11.889 (2) Å, c = 7.157 (1) Å, α = 98.28 (1)°, β = 105.25 (1)°, and γ = 60.04 (1)° at-117 (3) °C. The structure has been refined anisotropically by full-matrix least-squares techniques to conventional and weighted R indices of 0.049 and 0.084, based on 1978 observations (I ≥ 3σ(I)). There are number of unusual features associated with the structure. First, the dimers contain a weak Ni-Ni bond of length 3.063 (1) Å. Secondly, the macrocylic ligands of the dimers are eclipsed, maximizing rather than minimizing nonbonding interactions. Thirdly, the double bonds of the macrocyclic ligand are essentially delocalized. A qualitative bonding scheme is applied to account for these features.