STABILITY ORDER IN METAL CHELATE COMPOUNDS .V. BIVALENT METAL COMPLEXES OF 2-PYRIDYLMETHYL PHOSPHATE

The complex formation between 2-pyridylmethyl phosphate and the bivalent metal ions, Mg(II), Mn(II), Co(II), Ni(II), Cu(II), and Zn(II), was investigated by means of a potentiometric method. Where the chelate system became complicated by the formation of a protonated complex, a nonlinear least-squares method was successfully applied to the analysis of data with the aid of an electronic computer. The resulting stability order of the 1:1 complex with respect to the bivalent metals has been found to follow the sequence: Mg < Co ∼ Mn < Zn ∼ Ni ≪ Cu. The donor group of a significant coordination ability in this metal complex formation seems to be the phosphate group, and the pyridyl nitrogen plays merely a minor role in such a process. Meanwhile, the copper ion has been shown to form a chelate ring with the present ligand by involving both the pyridyl nitrogen and the phosphate group. A plausible structure of the copper chelate based on the stability data has been proposed. The formation of metal chelates of a 2:1 molar ratio of ligand to metal ion has not been detected under present experimental conditions.