A New Sulfide In Town

Chemists don’t take oxidation states lightly, and when someone suggests a new one might be in the offing, the extraordinary claim requires extraordinary proof. A research team led by John F. Berry of the University of Wisconsin, Madison, is now reporting its proof for a new oxidation state for sulfur, S₂^3– (J. Am. Chem. Soc. 2015, DOI: 10.1021/ja511607j). This subsulfide, as the researchers call it, joins the well-known sulfide, S^2–, and disulfide, S₂^2–, states. Berry says the initial suggestion for subsulfide, found in a transition-metal complex in which an S₂ unit coordinates to a pair of nickel atoms, was controversial. Nickel complexes containing sulfur, selenium, or tellurium (M₂E₂ units) were discovered in 2001 and found to have unusual bond distances. That led to some ambiguity as to how to best describe the compounds’ electronic structure. Three possibilities emerged: singly bonded S₂^2– units, three-electron half-bonded units that are formally S₂^3– with mixed-valent nickel atoms, and two independent S^2– units. The new paper provides crystallographic and spectroscopic evidence supported by computational analysis that all three states are distinct and that M₂E₂ compounds occur in quantized oxidation states, rather than displaying a continuum of E₂ bonding interactions.