ON THE PROTONATION EQUILIBRIA OF CYCLO-μ-IMIDO-POLYPHOSPHATE ANIONS (I)

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Protonation equilibria of trimeric cyclic anions, cyclo-triphosphate, cyclo-mono-μ-imido-triphosphate, cyclo-di-μ-imido-triphosphate, and cyclo-tri-μ-imido-triphosphate have been investigated potentiometrically at 25±0.5°C under N₂ atmosphere. The first protonation constants of the anions increase with an increase in the number of the imino groups which constitute the ligand molecules. The increase in the basicity of the ligands can primarily be interpreted by the difference in the electronegativity of the bridging oxygen and nitrogen atoms, however, tautomerism of the imidophosphate bonding are also considered to be responsible to the enhancement of the H⁺ ion binding ability of the imidopolyphosphate anions.