Stereoisomerism of bis(σ-Zincane) Complexes: Evidence for an Intramolecular Pathway.

The first bis(σ-zincane) complexes, heterotri- metallic species [M(CO)4 (η2 -HZnBDI)2 ], have been prepared (BDI=κ2 -{2,6-(iPr)2 C6 H3 NCMe}2 CH). For M=Cr, a single stereoisomer is observed in solution and the solid-state. For M=Mo and W, cis and trans isomers were found to reversibly interconvert at 297 K. Despite the huge steric demands of the ligand on zinc, the cis isomer was found to be the most thermodynamically stable in all cases. The activation parameters for the isomerisation when M=Mo are ΔH≠ =20.8 kcal mol-1 and ΔS≠ =-12.8 cal K-1  mol-1 . In combination with DFT calculations, the negative activation entropy suggests an intramolecular rotation mechanism for isomerisation.