An unexpectedly strong chelate effect in a cobalt half-sandwich complex with an intramolecularly coordinated C-C double bond

A pronounced chelate effect is observed in the cobalt half-sandwich complex 1, bearing an intramolecularly coordinated 3-butenyl side chain. The chelating C-C double bond is only slowly cleaved under ethylene pressure of 36 atm to form the bis(ethylene) complex 2. Two-electron ligands CO, P(OMe)₃, and dimethyl fumarate give chelate complexes 3-5, whereas the bidentate ligand 1,5-cyclooctadiene affords the "open-chain" complex 6. © 1992 American Chemical Society.