Biradical-type ligand

An important property of metal-bound phosphole ligands is their ability to undergo additional reactions not possible in the noncomplexed form. This is nicely illustrated by the thermally induced reactions of the palladium(ll) complex of 1-phenyl-3,4-dimethylphosphole 341 <1996IC1486>. Heating complex 341 at 145 °C in solution or at 140 °C in the solid state led to the formation of a mixed 7-phosphanorbornene-phosphole complex 343 (Scheme 114). These intramolecular [4-1-2] cycloaddition reactions are believed to proceed via the initial formation of a diallyl 1,4-biradical TS 342. Further examples of this type of reaction may be found in Section 3.15.12.1.1. 

Acetylenes are a priori electron-donating ligands with only weak acceptor properties and their n complexes should be relatively labile. The initial step of at least some of the reactions could be a conversion of the complexed acetylene in its ground state to a biradical with sp hybridized carbon atoms. This activated acetylene complex may then stabilize itself in many ways, depending largely on the type of the additional coordinated ligands in the transition state. 

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