The separation of phosphorus (III) ligands into two distinct groups identified as pure -donor ligands and -donor/tr-acceptor ligands for the acetyl and methyl complexes, (jj-Cp)FeL(CO)COMe, (rj-Cp')FeL-(CO)COMe (Cp' = MeC5H4), and (ij-Cp)FeL(CO)Me, is accomplished by correlation of the terminal carbonyl stretching frequencies with the EL0' values. The basicity (p value of HPR3+) is related primarily to the -donicity (the ability of a ligand to donate -electrons to a transition metal) and to a lesser degree to the size of the ligand. We conclude that p values are reasonable measures of the -donicity for those ligands that are pure -donor ligands; a better measure are the values for those ligands that are pure -donors for both the iron complex and LNi(CO)3.

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