Chapter 4, Problem 4.33P

Complexes of the general formula $\text{Fe}{\left(\text{CO}\right)}_{{\text{5}}^{-}\text{x}}{\left({\text{PR}}_{\text{3}}\right)}_{\text{x}}$ are long known. The bimetallic ${\text{Fe}}_{\text{2}}{\left(\text{CO}\right)}_{\text{9}}$ reacts with triphenylphosphine in refluxing diethyl ether to afford a monosubstituted product $\text{Fe}{\left(\text{CO}\right)}_{\text{4}}\left({\text{PPh}}_{\text{3}}\right)$ that exhibits (CO) absorptions at 2051, 1978, and 1945cm in hexane. (N. J.Farrer, R. McDonald, J. S. Mclndoe, Dalton Trans., 2006, 4570.) Can these data be used tounambiguously establish whether the ${\text{PPh}}_{\text{3}}$ ligand is bound in either an equatorial or axial site in this trigonalbipyramidal complex? Support your decision by determining the number of IR-activeCO stretching modes for these isomers.