Bis-phosphorus stabilised carbene complexes

The stabilisation of the carbene centre in a complex can be achieved either by the metal moiety or the carbon substituents. The balance between these two stabilising effects determines the nature of the M=C bond and therefore the reactivity of the metal complexes. Introducing two vicinal phosphorus groups as substituents of the carbene centre proved to be rewarding, since depending on the coordination number of this atom different electronic properties are observed. Indeed, a σ³-P atom possesses a lone pair that can interact with a carbene centre by destabilising its vacant p_π orbital. On the contrary a σ⁴-P group presents low lying σ* orbitals which can be involved in delocalising electronic density in the carbene p_π orbital by negative hyperconjugation. Therefore, PCP carbene complexes can exhibit either an electrophilic or nucleophilic reactivity depending on the nature of the phosphorus group and the metal centre. Carbenes complexes of early and late transition metals, but also of lanthanides are discussed in this perspective.