Potentiometric and spectroscopic studies of the cobalt(II), nickel(II) and manganese(II) complexes with some aminodiphosphonic acids in aqueous solution

The stoichiometries and stability constants of the manganese(II), cobalt(II) and nickel(II) complexes with seven aminodiphosphonates [RN(CH2PO3H2)(2)L] containing iminomethylenephosphonate moieties have been determined pH-metrically at 25 degreesC and at an ionic strength of 0.20 mol dm(-3) (KCL). The results suggest for L=1-6 only equimolar species: [M(H2L)], [M(HL)](-), [ML](2-) and hydroxo [MH-1L](3-), in the pH range of 3-11.5, the only difference is the presence of an additional [M(H3L)](+) species at low pH for N-3-picolyliminodi(methylenephosphonic) acid. For L=7 the models contain protonated and non-protonated 1:2 species. From the comparative analysis of the stability and the spectroscopic (UV-VIS) data it has been established that the ligands coordinate to the metal ions only by the phosphonate group(s) in a monodentate or bidentate manner. The LMCT bands from O-(PO32-) to Ni2+ are at 222-211 nm. (C) 2000 Elsevier Science Ltd. All rights reserved.