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] · 8H 20

Abstract

Potentiometric investigations in aqueous solutions at 25°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, H 4L) at the concentrations and ligand: metal ratio studied: ratio 1 : 1, complexes H i M n L n ( 2 n−2− 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 H n M n+1 L n (2 n−2− i−) , n = 1 for all cations and n = 2, 3 for Ni II and Co II; with excess of ligand, complexes H i M n L n+1 (2 n+ 4− i)− - with n = 1 mainly. The formation constants β 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 20 were obtained. X-ray diffraction structural analysis revealed that in the 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.