**Ante Miličević* and Nenad Raos**

*Institute for Medical Research and Occupational Health, Ksaverska c. 2, P.O.
Box 291, HR-10000 Zagreb, Croatia
* Corresponding author: E-mail: antem@imi.hr*

**Abstract**

Valence connectivity index ^{3}*χ*^{v} was correlated to the log
*K*^{1}(L) of copper(II)
complexes with diethylenetriamines (CuL, *N* = 8), and to logarithm of the second, log
*K*^{2}(L) and log *K*^{2}(A), and overall
stability constant, log *β*^{2}, of copper(II) mixed
complexes (CuLA, N = 18) with diethylenetriamines and amino acids. Regression on
log *K*_{1}(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 *K*_{1}(L) and log *K*_{2}(A), and 0.5 for log
*K*_{2}(L) and log *β*2.

**Keywords:** Diethylenetriamines, amino acids, mixed complexes, indicator variable