Importance of π-bonding in the stabilisation of ternary complexes

Summary Stability constants (K_MAL^MA) together with other thermodynamic parameters measuring the stabilities and the contribution of astatistical factors governing the stability of ternary complexes, MAL [M=Co^II, Ni^II, Cu^II or Zn^II; A=2, 2-dipyridylamine (A³), 5-nitro-o-phenanthroline (A⁵), 5-methyl-o-phenanthroline (A⁶); LH=benzohydroxamic acid] have been determined at 25°CC, at ionic strength 0.1 M KNO₃. The results are compared with those for the complexes containing polyaminocarboxylic acids such as iminodiacetic acid (A¹) and other heteroaromaticN-bases such as 2, 2-bipyridine (A²),o-phenanthroline (A⁴) and the stability constants are found in the sequence For all heteroaromaticN-bases, the sequence has been found. The sequences are explained in terms of electrostatic interaction, change of electrophilicity of the bound metal and the -acidic character of the primary ligand.