[LFe II ] + ←[Fe 0 (CO)2Cp] -. The availability of LFeR complexes in the literature, in which R can be a variety of ligands, allows us to investigate the classification of Fp -as a ligand. A particularly useful tool is the Mössbauer isomer shift, which gives information about the s-orbital electron density at the Fe nucleus. Figure displays a plot of the isomer shift of some known three-coordinate LFe II R complexes (L is any β-diketiminate ligand). Softer ligands (CR3, PR2) appear at lower isomer shifts than harder ligands (Cl, NR2). This is further corroborated by the phosphide series, which follows a Hammett relationship (Cy > iPr > Ph), with more strongly donating ligands at lower isomer shifts. On this scale, Fp falls in the region of phosphides and CPh3 in terms of ligand properties. A recent computational study on Ln-Fp bonding (Ln = lanthanide) concluded that Fp is comparable to iodide, I -. Our results indicate that in this high-spin iron(II) system, Fp - is a stronger donor than I -. Experimental details can be found in Table . Two isomer shift values for each Fp complex are given due to differing fit parameters.