Isocyano analogues of [Co(CO)(4)](n): a tetraisocyanide of cobalt isolated in three states of charge.

The encumbering m-terphenyl isocyanide ligand, CNAr(Mes2) (Mes = 2,4,6-Me(3)C(6)H(2)), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1-, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)(4)](-), Co(CO)(4), and [Co(CO)(4)](+). Sodium amalgam reduction of CoCl(2) in the presence of CNAr(Mes2) provides the salt Na[Co(CNAr(Mes2))(4)], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr(Mes2))(4). X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr(Mes2))(4) modulates between D(2d)- and C(2v)-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b(2)-symmetric C-Co-C bending modes. Treatment of Na[Co(CNAr(Mes2))(4)] with 2 equiv of FcOTf, followed by addition of Na[BAr(F)(4)], provides the salt [Co(CNAr(Mes2))(4)]BAr(F)(4), which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr(Mes2))(4)]BAr(F)(4) are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO)(4)](+).