Abstract:

Here we report electrochemical, spectroscopic, and crystallographic characterization of a redox series of cobalt complexes in five sequential oxidation states. A simple bidentate phosphine ligand, cis-1,2-bis(diphenylphosphino)ethylene (dppv), allows for isolation of the 3+, 2+, 1+, 0, and 1– oxidation states of cobalt─the only known example of transition-metal complexes with redox-innocent ligands in five oxidation states. Electrochemistry of [Co(dppv)₂]²⁺ reveals three reversible reductions and one reversible oxidation. Complexes in each oxidation state are characterized using single-crystal X-ray diffraction. The coordination number and geometry of the complex changes as a function of the oxidation state: including acetonitrile ligands, the Co³⁺ complex is pseudo-octahedral, the Co²⁺ complex is square-pyramidal, the Co⁺ complex is pseudo-square-planar, and the Co⁰ and Co^– complexes approach pseudo-tetrahedral, illustrating structures predicted by crystal-field theory of inorganic transition-metal complexes.

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