Structural, Spectroscopic, and Electrochemical Studies of the Complexes [Ni₂(μ-CNR)(CNR)₂(μ-dppm)₂]<i>ⁿ</i>⁺ (<i>n</i> = 0, 1, 2): Unusual Examples of Nickel(0)−Nickel(I) and Nickel(0)−Nickel(II) Mixed Valency

Reaction of Ni(COD)2 (COD = cyclooctadiene) with dppm (dppm = bis(diphenylphosphino) methane) followed by addition of alkyl or aryl isocyanides yields the class of nickel(0) dimers Ni2(μ-CNR)(CNR)2(μ-dppm)2 (R = CH3 (1), n-C4H9 (2), CH2C6H5 (3), i-C3H7 (4), C6H11 (5), t-C4H9 (6), p-IC6H4 (7), 2,6-(CH3)2C6H3 (8)). The cyclic voltammograms of the dimers exhibit two sequential single electron oxidations to the +1 and +2 forms. Specular reflectance infrared spectroelectrochemical (IRSEC) measurements demonstrate reversible interconversions between the neutral Ni(0) dimers and their +1 and +2 forms. Bulk samples of the +2 forms are prepared by chemical oxidation using [FeCp2][PF6], while the +1 forms are prepared by the comproportionation of neutral and +2 forms. The neutral complexes 6 and 8 were characterized by X-ray diffraction as symmetric, locally tetrahedral binuclear Ni(0) complexes. The +2 forms of these complexes, 62+ and 82+, have asymmetric structures with one locally square planar and one locally tetrahedral metal center, evidence for a Ni(II)−Ni(0) mixed valence state. The X-ray structural characterization of 6+ is symmetrical and qualitatively similar to that of the neutral complex 6. The +1 forms all exhibit intense near IR electronic absorptions that are assigned as intervalence charge transfer (IVCT) bands. On the basis of structural, spectroscopic, and electrochemical data, the +1 forms of the complexes, 1+−8+, are assigned as Robin−Day class III, fully delocalized Ni(+0.5)−Ni(+0.5) mixed valence complexes.