OCTAHEDRAL METAL-CARBONYLS .68. KINETICS AND MECHANISM OF THE DISPLACEMENT OF CHLOROBENZENE BY PIPERIDINE FROM THE PHOTOPRODUCED [CHLOROBENZENE-PENTACARBONYLCHROMIUM(0)] SOLVATE

Upon flash photolysis in chlorobenzene (CB) solution, Cr(CO)6 reacts with piperidine (pip) to afford (pip)Cr(CO)5. The reaction proceeds via photochemically-induced dissociation of CO and creation of specifically-solvated [(CB)Cr(CO)5]. Studies in CB/hex (hex = hexanes) solutions indicate that displacement of CB by pip proceeds via a rapid solvation/desolvation pre-equilibrium and attack by pip at [Cr(CO)5]. While the activation enthalpy for Cr-pip bond-fission is some 66.5. (29) kJ mol-1greater than for Cr-CB bond-fission, the “competition ratios” of rate constants for pip and CB attack at the reacting intermediate to afford [(pip)Cr(CO)5] and (CB)Cr(CO)5at 13.8-35.4°C are 1.37(8) and are temperature-independent. Based upon data from previous studies, these results indicate that [Cr(CO)5] rather than [(hex)Cr(CO)3] is the intermediate which reacts with CB and pip. It also may be concluded that the enthalpy of activation for CB-Cr bond-fission, 41.0 (4) kJ mol-1closely approximates the CB-Cr bond-strength. © 1990, Taylor & Francis Group, LLC. All rights reserved.