Solution structure and dynamic behaviour of two isomers of [Fe₃(CO)₉{P(OR₃)}₃] (R = Me, Et, or Ph) derivatives

The reaction between [Fe₃(CO)₁₂] and P(OR)₃ (R = Me, Et, or Ph) in the presence of the bimetallic catalyst [{Fe(CO)₂(PPh₃)(SEt)}₂] affords two 1,2,3-trisubstituted isomers which rapidly interconvert at room temperature on the n.m.r. time-scale. The solution structures and dynamics have been elucidated for both isomers by means of variable-temperature ³¹P and ¹³C n.m.r. studies. In polar solvents the major isomer shows a [Fe₃(CO)₁₂]-like structure whereas the minor one contains only terminally bonded CO groups. The observation of three equally intense carbonyl resonances in the low-temperature-limiting spectra (R = Me or Et) supports a D₃ structure for this isomer. The equilibration of axial CO ligands occurs readily as the temperature is increased; this rearrangement may be ascribed to the motion of the Fe₃ triangle inside the polyhedral cloud of ligands or alternatively to an interconversion between a left-hand screw with a right-hand screw.