Irving-Williams Order in the Framework of Connectivity Index (3)χ(v) Enables Simultaneous Prediction of Stability Constants of Bivalent Transition Metal Complexes

Logarithms of stability constants, log K(1) and log β(2), of the first transition series metal mono- and bis-complexes with any of four aliphatic amino acids (glycine, alanine, valine and leucine) decrease monotonously with third order valence connectivity index, (3)χ(v), from Cu(2+) to Mn(2+). While stability of the complexes with the same metal is linearly dependent on (3)χ(v), stability constants of Mn(2+), Fe(2+), Co(2+), and Ni(2+)complexes with the same ligand show a quadratic dependence on (3)χ(v). As Cu(2+) complexes deviate significantly from quadratic functions, models for the simultaneous estimation of the stability constants, yielding r = 0.999 (S.E. = 0.05) and r = 0.998 (S.E. = 0.11), for log K(1) and log β(2), respectively, were developed only for Mn(2+), Fe(2+), Co(2+), and Ni(2+) complexes with amino acids.