Preparation and Spectroscopic Characterization of rra/i5-Fe(CO)3L1L2 (L1, L2 = Phosphine or Phosphite)

The 57Fe M ößbauer spectra of mixed ligand com plexes o f the type frfl«5-Fe(CO)3L'L: (L 1 = triphenylphosphine or triphenylphosphite and L2 = phosphine or phosphite) show a quadrupolesplitting doublet typical of the disubstituted iron carbonyls in trigonal bipyramidal symmetry. The inverse linear dependence of the isomer shifts on the CO stretching frequencies is interpreted on the basis of the strengthening triple-bond nature o f the carbonyl ligands with increasing iron-tophosphorus jr-back donation. A linear correlation, with a positive slope, between the isomer shifts and the quadrupole splittings has revealed that the phosphorus-to-iron a-donation is offset by the iron-to-phosphorus jr-back donation. A correlation between the coordination shifts and the isomer shifts demonstrates that the iron-to-phosphorus jr-back donation plays an important role in the Fe —P bond. The relatively large coupling constant o f 2y(P ,P ) reflects a strong interaction between fra«5-phosphorus ligands through the P —Fe —P bond.