Spectrophotometric study of ion pairing in diphenylmethyl alkali metals salts

A spectrophotometric study of diphenylmethyllithium (DPM−Li+) and diphenylmethyl-potassium (DPM−K+) in ethereal solvents has yielded information on ion pairing and solvation phenomena in these carbanion systems. Different spectral absorptions are observed, characteristic of two types of contact ion pairs (unsolvated and partially solvated) as well as the solvent separated ion pair species, on varying the cation and solvent. This contrasts with our previous observations with triphenylmethyl alkali metal salts where only contact and solvent separated ion pairs were observed. The effect of 18-crown-6 polyether and the effect of temperature changes on the ion pairing equilibria are evaluated. Thermodynamic parameters are obtained for equilibria pertaining to the DPM−Li+/THF and DPM−Li+/DME systems. The results are discussed in relation to literature reports on ion pairing in these systems as derived from nmr studies. Comparison with triphenylmethyl alkali metal salts yields information relating to delocalization and steric effects on ion pairing.