Synthesis, molecular structure, and dynamic behaviour in solution of octakis(t-butyl isocyanide)dicobalt

Reduction of [Co(CNBu^t)₅][PF₆] with potassium amalgam in tetrahydrofuran affords red air-sensitive crystals of octakis(t-butyl isocyanide)dicobalt. Crystals of [Co₂(CNBu^t)₈] are orthorhombic, space group P2₁2₁2, with two molecules in a cell of dimensions a = 18.017(4), b = 12.149(4), and c = 11.382(5) Å. The structure has been solved by conventional techniques and refined by least squares to R 0.085 for 2 376 reflection intensities recorded on a four-circle diffractometer. The molecular geometry resembles that of [CO₂(CO)₈], but is substantially distorted, especially with respect to one of the basal isocyanide functions. Non-bonded contacts calculated for an idealized molecular geometry strongly suggest the importance of intermolecular contacts in the adopted distortions. The cobalt-cobalt distance is 2.4567(20) Å. The ¹³C n.m.r. spectrum of an enriched (10%) sample of [Co₂( CNBu^t)₈] shows one resonance at ambient temperature collapsing (coalescence temperature -60 °C) to three resonances (1:1:2) at -90 °C. Evidence is presented for an intramolecular exchange process. Reaction of [Co₂(CNBu^t)₈] with CO, NO, and diphenylacetylene affords respectively [Co(CNBu^t)₅][Co(CO)₄], [Co(CNBu^t)₃-(NO)], and [Co₂{μ-(η²-PhC₂Ph)}(CNBu ^t)₆].