²⁷Al NMR studies on the complexation of aluminum(III) with phosphinate and phosphite ions

The complex formation of aluminum(III) with phosphinate and phosphite ions in acidic solutions (pH 1-3) has been investigated by ²⁷Al NMR spectroscopy. The formation of a 2:2 complex was established in addition to ordinary stepwise complexes in both Al³⁺-PH₂O₂^- and -PHO₃^2- systems. Formation constants of these complexes formed in both systems, and thermodynamic quantities for the complexation in the Al³⁺-PH₂O₂^- system were determined at an ionic strength of 1.0 moldm^-3 NaClO₄. The stepwise complexation up to 1:3 becomes more exothermic and less entropy-productive in a regular manner. Both ΔH° and ΔS° values for the 2:2 complexation are quite large and positive due to the dimerization.