Complex Formation Equilibria between Ag(I) and Thioureas in Propan-2-ol

The complex formation equilibria between Ag(I) and thiourea (Tu), N methyl-Thiourea (MeTu), N,N′-dimethyl-Thiourea (Me2Tu), N,N,N′,N′ Tetramethyl-Thiourea (Me4Tu), N-ethyl-Thiourea (EtTu), N,N′ diethyl-Thiourea (Et2Tu), and N,N′-Ethylen-Thiourea (EnTu) in propan-2-ol as solvent medium have been investigated by potentiometry within the temperature range 10°C–40°C and self-determined ionic medium by (AgNO3) concentration range m·dm？3 (no background electrolyte added). The experimental results can be interpreted with the stepwise formation of tris-coordinated complexes ( ; hereafter charge is omitted). The stability constants log and the standard thermodynamic changes in enthalpy ( ) and entropy ( ) for overall complex formation reactions ( ) have been evaluated. The reactions are exothermic in nature and entropy disfavored for all the investigated ligands. The standard enthalpy and entropy changes for the stepwise reactions have been evaluated by difference and . The stepwise enthalpy are mutually linearly related to the corresponding entropy changes with isoequilibrium temperatures , 305, and 363？K for , 2, or 3 respectively. 1. Introduction In homogeneous equilibria, the solvent medium operates simultaneously on both reactants and products and influences the chemical reactions with counteracting actions which originate from solute-solvent and solvent-solvent interactions. The quantitative balance of such interactions together with the solute-solute interactions determines the actual position of the chemical equilibrium [1–5]. Systematic investigations on single series of reactions but different solvents are of utmost importance for disclosing the role of both the solvents and the various elements of the series on the metal-to-ligand interactions. Thioureas and in general thiocarbonyl compounds are soft ligands which in hard solvents coordinate to soft metal ions via sulfur atom(s) [6, 7]. They show relevant chemical affinities towards Ag(I) forming in hydroxyl solvents complexes with high thermodynamic stability ([8, 9] and cited references). In hydrogen donor solvents, thioureas undergo simultaneous solvation both at the sulfur atom via the acceptor atom(s) of the solvent and, when not exhaustively N-alkylated, to the hydrogen atoms via the donor atom(s) of the solvent. In hard solvents, in comparison with other metal ions in their ordinary oxidation states, silver ion has remarkable oxidizing properties which suggest that Ag(I) undergoes no strong solvation in such solvents. The coordination of thiocarbonyl compounds to soft metal ions occurs via