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) exuists as two isomers in solution. One has the PEt3 ligand axial to the bridged face of the iridium tetrahedron and thre second minor isomer has the PEt3 ligand equatorial. This gives rise to fourteen different carbonyl signals. Magnetization 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 40 kJ mol-1. This clearly shows that neither the bridged or unbridged forms represent the transition state.
|The all terminal structure of Ir4(CO)11L||The bridged structure of Ir4(CO)11L|