The rearrangement pathway in [Cp2Mo2(CO)4(RCCCR2)]+ cations: An extended Hückel molecular orbital and Bürgi-Dunitz trajectory study

The fluxional behavior of metal cluster cations of the type [Cp2Mo2(CO)4(RCCCR′R″)]+ has been widely studied by variable-temperature NMR techniques, and a mechanism involving migration of the carbocationic center between the organometallic vertices has been proposed. In this report, the suggested mechanism has been investigated by means of extended Hückel molecular orbital calculations (EHMO), and allows the evaluation of the lowest energy pathway by which this process may occur. These computations are supplemented by a Bürgi-Dunitz trajectory analysis of 11 X-ray crystal structures of such cations which yields a series of “snapshots” of the migration process. Together the EHMO and trajectory approaches give a remarkably complete picture of the molecular dynamics of such molecules.