Polymer species in aqueous solutions of para-phenylenediamine-N,N,N',N'-tetraacetic acid (p-PhDTA) with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II). X-ray crystal structure of Na-4[Co-2(p-PhDTA)(2)]center dot 8H(2)O

Potentiometric investigations in aqueous solutions at 25 degrees C and ionic strength 0.1 (Cu-II, Ni-II, Co-II, Zn-II and Cd-II) and 0.5 mol dm(-3) KCl (Cu-II) analysed by LETAGROP and simulation calculus for a three-component system [Z(pH) curves] show that p-phenylenediamine-N,N,N',N'-tetraacetic acid (p-PhDTA) forms protonated and non-protonated monomer and polymer complexes (ligand, H4L) at the concentrations and ligand:metal ratio studied: ratio 1:1, complexes HiMnLn(2n-i-), n = 1-3,5,6 (Cu-II, Ni-II and Co-II) and n = 1-3 (Zn-II and Cd-II); with excess of metal complexes HiMn+1Ln2n-2-i-, n = 1 for all cations and n = 2, 3 for Ni-II and Co-II; with excess of ligand, complexes HiMnLn+1(2n+4-i-), with n = 1 mainly. The formation constants beta(pqr) of the complexes have been determined and the Irving-Williams order of complexation is fulfilled. The species distribution diagrams indicate that the more important non-protonated polymer complexes are: trimer and pentamer for Cu-II; pentamer for Ni-II; dimer and trimer for Co-II; trimer for Zn-II and Cd-II. Co-II, Ni-II and Cu-II also form hexamers. From a concentrated solution with a ligand:metal ratio 2:3 at pH 5, single crystals of the complex Na-4[Co-2(p-PhDTA)2]. 8H(2)O were obtained. X-ray diffraction structural analysis revealed that in the dimer anion [Co-2(p-PhDTA)(2)](4-) the cobalt atoms are hexa-coordinated with each metal surrounded by four carboxylic oxygens and two amine nitrogens (from two iminodiacetate groups) in a distorted octahedron. Each sodium is coordinated to water molecules and carboxyl groups, being five-coordinated. (C) 1997 Elsevier Science Ltd.