Activation of the catalytically relevant complex [Cr(CO)4(1)]+ (1 = Ph2P(C3H6)PPh2) by Et3Al (TEA) leads to formation of the Cr(I) bis-arene complex [Cr(1-bis-η6-arene)]+, as revealed by EPR and DFT calculations. This bis-arene complex is formed by intramolecular rearrangement and coordination of Cr(I) to the ligand phenyl groups in aliphatic solvents following loss of CO, preventing release of Cr(I) into solution. By comparison in aromatic solvents (toluene), the [Cr(bis-tolyl)]+ complex is preferentially formed.

Bicyclic expanded ring carbenes show increased sterics compared to similar monocyclic derivatives. The enhancement is a consequence of an impacting methyl substituent.