Metal-alkenes and Alkyne Complexes
As in metal-carbonyls, electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of the alkenes and alkynes. This electron-transfer (i) strengthens the metal-ligand bond and (ii) weakens the C-C bonds within the ligand. In the case of metal-alkenes and alkynes, the strengthening of the M-C2R4 and M-C2R2 bond is reflected in bending of the C-C-R angles which assume greater sp3 and sp2 character, respectively. Thus strong pi-backbonding causes a metal-alkene complex to assume the character of a metallacyclopropane. Electronegative substituents exhibit greater pi backbonding. Thus strong pi backbonding ligands are tetrafluoroethylene, tetracyanoethylene, and hexafluoro-2-butyne.
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