Issue Date: April 20, 2015
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, S23– (J. Am. Chem. Soc. 2015, DOI: 10.1021/ja511607j). This subsulfide, as the researchers call it, joins the well-known sulfide, S2–, and disulfide, S22–, 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 S22– units, three-electron half-bonded units that are formally S23– with mixed-valent nickel atoms, and two independent S2– 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.
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