Vìsnik Odesʹkogo Nacìonalʹnogo Unìversitetu: Hìmìâ (Mar 2020)
RELATIONSHIP OF ACIDITY-BASICITY OF SALTS OF 3d -METALS WITH THEIR ABILITY FOR FORMATION OF AQUA (AMMIN) COMPLEX COMPOUNDS
Abstract
The general nature of the relationship between the acid-base properties (electronegativity) of 3d-metal salts and their ability to form aqua and ammine complex compounds (hydrates and ammoniates) has been established. The thermal strength of metal-ligand bonds (H2O, NH3), defined as the enthalpy of formation of a complex compound per bond depends on many factors: the coordination number of the central metal ion, the nature of the metal, anion, and ligand. An increase in the electronegativity of compounds in the series nitrates – chlorides – sulfates of 3d-metals (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, as well as its decrease when H2O is replaced by NH3 (with the same coordination number) results in an increase in strength by a decrease in enthalpy per 1 metal-ligand bond of complex compounds. In turn, the change in electronegativity is due to the size-charge parameters (metal ionic radius, counter ion charge) of the cations and anions that make up the compounds. There is a correspondence in the change in the total enthalpy of formation of aqua complex compounds (crystalline hydrates) and their diluted solutions. An explanation of this relationship is proposed, based on the recognition of the important role of the chemical component in the process of dissolution of compounds in water. Similar, albeit in a less pronounced form, processes take place in 3d-metal hydroxides, which have mainly basic properties. When water molecules are replaced by ammonia molecules, the solubility of hydroxides in the series Mn(OH)2, Fe(OH)2, Co(OH)2, Ni(OH)2, Cu(OH)2, Zn(OH)2 significantly increases, indicating, together with a change in color, the strengthening of complex compounds. Found regularities make it possible to use the electronegativity parameter to predict and control complexation in other groups of compounds (salts, hydroxides, and molecular ligands).
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