Phosphorus-31 contact shifts as a measure of weak ligand affinities. Interaction between alkali metal fluorenone radical anions and certain phosphorus(III or V) ligands

Abstract

The nuclear magnetic resonance shifts, δobs, induced to 31P in Ph3P, (MeO)3P and (MeO)3PO by alkali metal fluorenone radical anions in tetrahydrofuran solutions were measured. These parameters, which depend significantly on the cation for a given ligand, and also on the ligand for a given cation, were used to extract the corresponding 31P contact shifts, δc(M). The latter parameters are proposed as a measure of the ligand affinity of a given phosphorus compound for a particular alkali metal. Trimethyl phosphate exhibits the greatest affinity for the lithium cation. The magnitudes of δc(M) imply some covalency between the cation and the radical anion and the ‘paradox’ of a heavier alkali metal such as potassium being covalently bonded to fluorenone radical anion has been explained. The transferability of the reported ligand affinities of Li+, Na+ and K+ is discussed. © John Wiley & Sons, Ltd.