Irving-Williams order in the framework of connectivity index ³χv enables simultaneous prediction of stability constants of bivalent transition metal complexes.

Logarithms of stability constants, log K₁ and log β₂, 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, ³χv, from Cu²+ to Mn²+. While stability of the complexes with the same metal is linearly dependent on ³χv, stability constants of Mn²+, Fe²+, Co²+, and Ni²+complexes with the same ligand show a quadratic dependence on ³χv. As Cu²+ 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 K₁ and log β₂, respectively, were developed only for Mn²+, Fe²+, Co²+, and Ni²+ complexes with amino acids.