BOND CHARACTER BETWEEN IRON AND PHOSPHORUS IN FE-P(E)YZ (E = O, S - Y, Z = ALKOXY, AMINO, PHENYL) AS INFERRED FROM FE-57 MOSSBAUER MEASUREMENTS

A series of (eta(5)-C5R5)(CO)(2)Fe{P(E)YZ} complexes (R = H, Me; E = O , S; Y, Z = OMe, OEt, NEt2, MeNCH2CH2NMe, Ph), (eta(5)-C5H5)(CO)(2)FeX (X = Cl, Br, I), and (eta(5)-C5Me5)(CO)(2)FeCl were prepared, and the Fe-57 Mossbauer spectra together with IR(nu(CO)) and P-31 NMR spectra were measured. The Fe-P bond in (eta(5)-C5R5)(CO)(2)Fe{P(E)YZ} is fou nd to be as covalent as the Fe-Me bond in (eta(5)-C5H5)Fe(CO)(2)Me. Th ere are linear correlations between isomer shifts (IS) and quadrupole splittings (QS) in the Fe-57 Mossbauer spectra, between IS and chemica l shifts in the P-31 NMR spectra (delta), between IS and the IR stretc hing frequencies in the CO region (nu(CO)), and between delta and nu(C O). These correlations indicate that the electron-donating group on th e phosphorus atom (Y, Z) increases the electron density on the phospho rus atom, increases pi-donation from P(E)YZ to the Fe, and concomitant ly increases pi-back-donation from iron to the CO ligands. The systema tic replacement of the carbonyl group by phosphine or phosphites shows the large shift in IS and QS, indicating a large change in the d-elec tron density of the iron atom. Especially for the complexes having no carbonyl ligand, the d-electron density is much accumulated on the iro n atom.