N-heterocyclic carbenes are a well-known class of complexes and have met with great success because of their stability and their easy and versatile synthesis. Their phosphorus analogues have received comparably less attention; however, the introduction of phosphorus containing groups can be foreseen to be rewarding due to the ability of the phosphorus atom to adopt various coordination numbers that dramatically affect its electronic properties.

In this Dalton Transactions Perspective, Pascal Le Floch and colleagues at the Ecole Polytechnique, France  discuss recent works devoted to the study of carbene complexes bearing two phosphorus groups on the carbon atom.

The authors present the two different strategies devised: sigma³-P and sigma⁴-P. The electronic properties of these moieties are opposing: a sigma³-P atom possesses a lone pair and therefore acts as a pi donor, whereas a sigma⁴-phosphorus fragment presents low lying sigma* orbitals and therefore acts as a pi acceptor. The chemical behaviour of the carbene may therefore be tuned from nucleophilic to electrophilic by changing the substituents at P and the metal centre. 

N-heterocyclic carbene vs P-heterocyclic carbene