Phosphorus and proton nuclear magnetic resonance studies of transition-metal complexes of triphosphate and pyrophosphate in aqueous solution
Abstract
The structures of transition-metal complexes of triphosphate and pyrophosphate have been examined in aqueous solution by phosphorus 31P and water proton n.m.r. For pyrophosphate, 1:1 bidentate complexes containing four water molecules are formed with Ni2+, Co2+ and Mn2+ in the pH range 7–10, but the Cu2+ complex contains two pyrophosphate molecules. 1:1 complexes are also formed between triphosphate and Ni2+, Co2+ and Mn2+ but there exists a dynamic equilibrium between bidentate and tridentate forms, the former being favoured by Mn2+ and the latter by Ni2+ and Co2+. A further equilibrium involving a complex containing two triphosphate molecules becomes important for Cu2+ and this predominates in alkaline media. The lifetimes for triphosphate bound to transition metals, Cu2+, Mn2+, Co2+, Ni2+ and Fe3+, have been determined as a function of temperature, and activation energies for ligand exchange are 4.6, 23.4, 32.1, 34.4 and 25.8 kJ mol–1, respectively. The rotational correlation times determined for triphosphate complexes of Mn2+ and Cu2+ are 5.6 × 10–10 and 4.0 × 10–10 s determined from 31P n.m.r., but the effective correlation times determine from water-proton n.m.r. are 4.5 × 10–11 and 5.0 × 10–11 s, respectively. The electronic relaxation time for Mn2+ decreases from 1.5 × 10–8 to 5 × 10–9 s on complexation with equimolar amounts of tripolyphosphate.