Determination of the Field Strengths of Weak Neutral Ligands Having Oxygen as Coordination Atom

Weakness of various neutral ligands having oxygen as coordination atom was ranked on the basis of the pyrrole proton chemical shifts of planar [Fe(TPP)L2]+. Furthermore, weakness of the ligands, which is difficult to determine from the NMR data of planar [Fe(TPP)L2]+, was successfully differentiated by the use of highly ruffled [Fe(TiPrP)L2]+. Among the 10 ligands examined, THF turned out to be the weakest ligand.