X-ray absorption spectroscopic, crystallographic, theoretical (DFT) and chemical evidence for a chalcogen-chalcogen two-center/three-electron half bond in an unprecedented "subselenide" Se2(3-) ligand.

Doing things by halves: The dimeric compound (Cp'Ni)(2)(μ(2)-Se(2)) (Cp' = 1,2,3,4-tetraisopropylcyclopentadienyl), shown in the scheme, was investigated by using low temperature X-ray crystallography and X-ray absorption spectroscopy. The Se K-edge energy strongly indicates a Se physical oxidation state of -1.5, consistent with an unprecedented two-center/three-electron half-bonded Se(2)(3-) or "subselenide" ion.