Nature of the complexes formed by alkali metal halides with phosphine oxides

Reaction of alkali metal halides (MX) with methylenediphosphine oxides and various related compounds in nonaqueous solutions leads to the formation of complex compounds. The compositions, properties, and stabilities of these compounds, which have been studied in detail in acetonitrile, are determined by the nature of the cations and anions of the alkali metal halides. Formation of neutral complexes with the composition [MX · L] and cationic complexes with the composition [ML]⁺ has been established. The most characteristic representative of complexes of the first type is [NaI · L]; in the complexes studied, L=R₂P(O)CH₂P(O)R₂ (R=Bu, BuO, or Ph), Ph₂P(O)CH₂P(O) (OC₂H₅)CH₂P(O)Ph₂ and (p-OCH₃C₆H₄)₂P(O)CH₂P(O)(C₆H₄CF₃-p)₂. Compound [LiL]⁺ is characteristic of complexes of the second type; the compounds containing Ph₃P(O), Ph₂P(O)CH₂P(O)Ph₂, and Ph₂P(O)CH₂P(O)(OC₂H₅)CH₂P(O)Ph₂ as ligands have been studied. Stability constants of the complexes [NaI · L] and [LiL]⁺ have been determined by measuring the dependence of the electrical conductivity of solutions of the alkali metal halides in acetonitrile on the concentration of the ligands. The complex-forming power of phosphine oxides increases with increase in the number of P=O groups. Stabilities of the complexes [NaI · L] with ligands with identical structure decrease with increase in the electronegativity of the substituents on the phosphorus atoms.