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.