Bond character between iron and phosphorus in Fe-P(E)YZ (E = O, S; Y, Z = alkoxy, amino, phenyl) as inferred from⁵⁷Fe mössbauer measurements

A series of (η⁵-C₅R₅)(CO)₂Fe{P(E)YZ} complexes (R = H, Me; E = O, S; Y, Z = OMe, OEt, NEt₂, MeNCH₂CH₂NMe, Ph), (η⁵-C₅R₅)(CO)₂FeX (X = Cl, Br, I), and (η⁵-C₅Me₅)(CO)₂FeCl were prepared, and the ⁵⁷Fe Mössbauer spectra together with IR(ν_CO) and ³¹P NMR spectra were measured. The Fe-P bond in (η⁵-C₅R₅)(CO)₂Fe{P(E)YZ} is found to be as covalent as the Fe-Me bond in (η⁵-C₅H₅)Fe(CO)₂Me. There are linear correlations between isomer shifts (IS) and quadrupole splittings (QS) in the ⁵⁷Fe Mössbauer spectra, between IS and chemical shifts in the ³¹P NMR spectra (δ), between IS and the IR stretching frequencies in the CO region (ν_CO)> and between δ and ν_CO These correlations indicate that the electron-donating group on the phosphorus atom (Y, Z) increases the electron density on the phosphorus atom, increases π-donation from P(E)YZ to the Fe, and concomitantly increases π-back-donation from iron to the CO ligands. The systematic replacement of the carbonyl group by phosphine or phosphites shows the large shift in IS and QS, indicating a large change in the d-electron density of the iron atom. Especially for the complexes having no carbonyl ligand, the d-electron density is much accumulated on the iron atom.