Phosphinines are planar, aromatic systems containing a sp²-hybridized phosphorus atom - phosphorus analogues of pyridine. Though first described in the '60s, they have only been explored as potential ligands in homogeneous catalysis over the last decade. But what has been found so far suggests the ligands have great potential.

In their Dalton Transactions Perspective, Christian Müller and Dieter Vogt from Eindhoven University of Technology in The Netherlands review what is known about phosphinines. They believe the heterocycle can "serve as the basis for the development of very promising new classes of phosphorus containing ligand systems for homogeneous catalysis".

Phosphinines have two different potential modes of coordination (the aromatic pi-system and the lone pair of electrons on the phosphorus atom) and are easily functionalized. So the properties of the ligands can be tailored for specific purposes.

Recently, rhodium complexes of phosphinines have been shown to be very active hydroformylation catalysts.   And the use of chiral phosphinines in asymmetric homogeneous catalysis is promising. The potential of these ligands is largely unexplored but Müller and Vogt say that "further investigation remains an exciting, challenging and rewarding research field".