The Intricate Chemistry of Cobalt(II) Halides and Bis(diphenylphosphanyl)ethane

Phosphane complexes of the putative formula [CoX2(dppe)] [X = Cl, Br, I; dppe = bis(diphenylphosphanyl)ethane] are frequently used catalyst precursors in cobalt-catalyzed reactions. Despite their widespread use, these complexes have not been structurally characterized. We demonstrate that synthetic procedures for [CoX2(dppe)] afford complexes of the formula [Co4X8(dppe)(5)] instead. Crystallographic analyses of these complexes reveal an ionic structure with pentacoordinate [CoX(dppe)(2)](+) cations and a dinuclear [Co2X6(dppe)](2-) anion. The bromido and iodido complexes crystallize in two forms, which differ in the geometry of the cation (square-pyramidal vs trigonal-bipyramidal).