Potentiometric determination of the formation constants for complexes of 3,2′,2″-triaminopropyldiethylamine with cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II)

The tripodal tetraamine ligand N{(CH2)3NH2}{(CH2)2NH2}2 (pee), has been investigated as an asymmetrical tetraamine chelating agent for CoII, NiII, CuII, ZnII and CdII. The protonation constants for this ligand and the formation constants for its complexes have been determined potentiometrically in 0.1 M KCl at 25 °C. The successive protonation constants (log Kn) are: 10.22, 9.51, 8.78 and 1.60 (n = 1–4). One complex with formula M(pee)2+ (M = Co, Ni, Cu, Zn and Cd) is common to all five metal ions and the formation constant (log βML) is: 12.15, 14.17, 16.55, 13.35 or 9.74, respectively. In addition to the simple complexes, CoII, CuII and ZnII also give hydroxo complexes, and CuII and NiII give complexes with monoprotonated pee. [Zn(pee)](ClO4)2 and [Cd(pee)Cl](ClO4) complexes were isolated and are believed to have tetrahedral and trigonal-bipyramidal structures, respectively.