Estimation of Stability Constants of Mixed Copper(II) Chelates Using Valence Connectivity Index of the 3rd Order Derived from Two Molecular Graph Representations

Valence connectivity index 3Xv was correlated to the log K1(L) of copper(II) complexes with diethylenetriamines (CuL, N = 8), and to logarithm of the second, log K2(L) and log K2(A), and overall stability constant, log Beta 2, of copper(II) mixed complexes (CuLA, N = 18) with diethylenetriamines and amino acids. Regression on log K1(L) strongly indicates that influence of alkylation at middle nitrogen atom differs from the influence of alkylation at terminal nitrogens. Thereby, in models for estimation of all stability constants we introduced an indicator variable, In; In = 1 for a complex with triamine alkylated at the middle nitrogen, otherwise In = 0. Also, we developed second set of models assuming there is no bonding between Cu(II) and middle N atom. Both approaches gave nearly the same standard error of estimate, i.e. 0.3 for log K1(L) and log K2(A), and 0.5 for log K2(L) and log Beta 2.