Fluxionality of Ir₄(CO)₁₂

One of the major mechanisms of fluxionality of Ir₄(CO)₁₂ is the merry-go-round mechanism, which was first described by Cotton for Co₄(CO)₁₂.

In 1989, we showed that Ir₄(CO)₁₁(PEt₃) exists as two isomers in solution. One has the PEt₃ ligand axial to the bridged face of the iridium tetrahedron and the second minor isomer has the PEt₃ ligand equatorial. This gives rise to fourteen different carbonyl signals. Magnetisation transfer was used to monitor the exchange pathway and to determine the rates.

It is commonly assumed that either the bridge-opened or the bridge-closed form represents the transition state of the dynamic process. In the cases of Ir₄(CO)₁₁(PH₂Ph) and Ir₄(CO)₁₁(PHPh₂) both isomers are found to be present in solution and only differ in energy by a couple of kJ mol^-1. The two isomers interconvert with an activation energy of the order of 40kJ mol^-1. This clearly shows that neither the bridged or unbridged forms represent the transition state.

References

F. A. Cotton, Inorg. Chem., 1966, 5, p. 1083.

B. E. Mann, B. T. Pickup, and A. K. Smith, "The mechanisms of fluxionality of [Ir₄(CO)₁₁(PEt₃)]: A reappraisal of the mechanisms of carbonyl fluxionality on clusters", J. Chem. Soc., Dalton Trans., 1989, pp. 889-893.

B. E. Mann, M. D. Vargas, and R. Khattar, "The synthesis and mechanisms of fluxionality of [Ir₄(CO)₁₁(PH_3-nPh_n)], n = 1 or 2", J. Chem. Soc., Dalton Trans., 1992, pp. 1725-1728.