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

Summary

Stability constants of binary (ML, ML₂) and ternary (MAL) complexes [M=Co^II, Ni^II, Cu^II or Zn^II; 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 K ^MA_MAL from K ^M_ML and K ^ML₂ while in the case of heteroaromaticN-bases as primary ligands, the values of K ^MA_MAL are very close to those of K ^M_ML . In the ternary systems studied, the values of K ^MA_MAL are in the sequence, K ^M(o-phen)_M(o-phen) >K ^M(bipy)_M(bipy)L ≫K ^M(ida)_M(ida)L >K ^M(Me-ida)_M(Me-ida)L >K ^M(nta)_M(nta)L >K ^M(ada)_M(ada)L , while in the case of Cu^II, the values of M ^M(nta)_M(nta)L and K ^M(ada)_M(ada)L are drastically reduced compared to all other primary ligands. For aminopolycarboxylic acids, the sequence of K ^MA_MAL is opposite to those of K ^M_MA and K ^M_MAL though in the sequence of K ^M_MA , K ^M_MAL and K ^MA_MAL 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.