sigma-->sigma* Transitions in Mn2(CO)8L2 complexes (L = P-donor ligands): pi-acidity and oxygen pendant group effects in phosphite ligands.

Comparison of the energies assigned to the sigma-->sigma* transitions in bis-axial Mn2(CO)8L2 complexes when L = phosphites and phosphinites with those when L = alkyl and aryl phosphines shows that the oxygen-containing ligands do show effects associated with their pi-acidity. However, the data also strongly suggest that the energies are affected by the presence of the oxygen atoms themselves to an extent that increases linearly with the number of oxygen atoms connected to the P-donor atoms. This "oxygen effect" is analogous to, but distinct from, the aryl effect that is also simply proportional to the number of aryl groups attached to the P-donor atoms. This resolves the incongruity of the claim that phosphites also show an "aryl effect", and suggests that specific "pendant group effects" due to the nature and number of atoms attached to the phosphorus atoms of P-donor ligands (aryl, oxygen, nitrogen, sulfur etc.) may be quite general.