Credit: © 2009 ACS

There has been only one example reported so far of a terminal phosphorus monoxide complex. The chemistry of this functional group, therefore, is unexplored compared with others containing main-group elements. Now, Christopher Cummins and Nicholas Piro of the Massachusetts Institute of Technology have used a terminal phosphorus monoxide molybdenum complex to create1 a compound with an unusual phosphorus–phosphorus double bond.

The starting complex reacted quickly with a silylphosphinidene complex — known to be a good phospha-Wittig reagent — to give the product. The resulting Mo–P and P–P distances are both very short, indicating two double bonds in a three-centre π-system, and the structure is described as 'singly bent'. This geometry is quite different from the 'doubly bent' system of bonds with longer distances, indicating single bonds with a lone pair at the α-phosphorus atom.

The complex was relatively unstable, reacting over hours or days to give phosphinidene trimers and tetramers. This is in contrast to the stability of equivalent Mo–N–N and Mo–P–N compounds. The unusual disphosphenido ligand acts as a three-electron donor, and thus confers different reactivity than more usual one-electron donors that are nucleophilic at the α-phosphorus atom.