Credit: © 2009 Wiley

Compounds with a carbon–sulfur triple bond are extremely rare, and those that have been proposed to exist are either highly unstable (CS) or with bonding deemed 'unclassical' (F3S–C≡SF3 and F5S–C≡SF3).

So Peter Schreiner, Grzegorz Mloston and colleagues, of the Justus Liebig University in Giessen and the University of Lodz in Poland, were surprised when H2C=S=O photochemically rearranged1 to give not the carbene they were hoping for, but H–C≡S–OH. The infrared spectrum of the product very closely resembled that given by high-level calculations. Further computational work to investigate the possible structure of this molecule suggested that the all-cis configuration was most likely. Even lower in energy, however, was the sulfinyl carbine species H–C–S–O–H, but this was not observed experimentally.

The length of the carbon–sulfur bond was calculated to be very similar to the triple bond in CS, and to the sum of the triple-bond covalent radii for carbon and sulfur. Bond-order analysis suggested that it is below three, so the team describe it as a weak triple bond or a strong double bond.