Stability and structure of ethylenedinitrilopoly(methylphosphonate) complexes of the divalent transition metal ions in aqueous solution

The formation and protonation of the complexes of a series of ethylenedinit rilopoly(methylphosphonic acids) (EDMP) [(Me)(2-p)(H2O3PCH2)NC2H4N(Me)(2-q) (CH2PO3H2)(q), p = 0-2, q = 1-2] With divalent transition metal ions (M = M n2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+) were investigated by means of potentio metry and P-31 NMR spectroscopy at 25.0 degreesC. The complex formation con stants and protonation constants of these complexes were determined by pH t itration. The P-31 NMR spectra of zinc and cadmium complexes were measured as a function of the pH, and the P-31 NMR chemical shifts of each chemical species were evaluated by using the equilibrium constants determined by pH titration. The results for EDMP complexes were compared with those of amino poly(methylphosphonate) (NMP) [(CH3)(3-r)N(CH2PO3H2)(r), r = 1 - 3] complex es and alkaline earth metal EDMP complexes. The stability constants of the metal complexes increase upon increasing the number of the phosphonate grou ps and are around the same between the EDMP and NMP complexes of a given me tal ion having the same total number of coordinating atoms. The stabilities of the edtmp (p = q = 2) complexes are around the same as those of the med tmp (p = 1, q = 2) complexes. These results reveal the structures in which two nitrogen atoms of the ethylenedinitrilopoly(methylphosphonate) co-ordin ate to the transition metal ion in any complex. These structures are differ ent from those of the corresponding alkaline earth metal complexes. The str uctures of the protonated complexes were estimated from the results of the protonation constants and the P-31 chemical shifts of the complexes.