Effect of Ionic Strength on the Stability and Thermodynamic Properties of Rhenium(V) Complexes with N-Ethylthiourea

The complexation in the [ReOCl₅]^2––Ox/Red–HCl system (Ox is bis-N-ethylformamidine disulfide (H₂DEfds²⁺), and Red is N-ethylurea (Etu)) was studied by potentiometric titration in the temperature range of 273.1–338.1 K, and the \(\log {{K}_{i}}\) values of the complexes were determined. The thermodynamic values of this process (ΔH⁰, ΔG⁰, and ΔS⁰) were calculated using the \(\log {{K}_{i}}\) values. The process exothermicity was found to decrease with an increase in the number of coordinated ligand molecules. The most favorable condition for the formation of stable complexes is the concentration of 4 M HCl; i.e., when the concentration of hydrochloric acid decreases from 6 to 4 M HCl, the stability of the resulting complexes increases.