Astatistical aspects of the stabilities of ternary complexes of cobalt(II), nickel(II), copper(II) and zinc(II) involving aminopolycarboxylic acids and heteroaromaticN-bases as primary ligands and acetohydroxamic acid as a secondary ligand

Abstract

Stability constants of binary (ML, ML2) and ternary (MAL) complexes [M=CoII, NiII, CuII or ZnII; A=iminodiacetic acid (ida),N-methyliminodiacetic acid (Me-ida), anthranilatediacetic acid (ada), nitrilotriacetic acid (nta), 2,2′-bipyridine (bipy), orthophenanthroline (o-phen); HL =acetohydroxamic acid] have been determined at 25°C at an ionic strength of 0.1M KNO3 by the Iriving Rossotti technique. In the case of aminopolycarboxylic acids as primary ligands, there is always a lowering of KMALMA from KMLM and K2ML while in the case of heteroaromaticN-bases as primary ligands, the values of KMALMA are very close to those of KMLM. In the ternary systems studied, the values of KMALMA are in the sequence, KM(o-phen)M(o-phen)>KM(bipy)LM(bipy)≫KM(ida)LM(ida) >KM(Me-ida)LM(Me-ida)>KM(nta)LM(nta)>KM(ada)LM(ada), while in the case of CuII, the values of MM(nta)LM(nta) and KM(ada)LM(ada) are drastically reduced compared to all other primary ligands. For aminopolycarboxylic acids, the sequence of KMALMA is opposite to those of KMAM and KMALM though in the sequence of KMAM, KMALM and KMALMA for A=ada and nta their relative positions are unaltered. The obtained results are explained in the light of different astatistical factors such as electrostatic effects, steric hindrance, change of effective positive charge on the central metal depending upon the σ-basic and π-acidic character of the primary ligands.