Solution equilibria of gallium(III) and indium(III) complexes with halide and thiocyanate ions in N,N-dimethylformamide

Abstract

Complexation of gallium(III) and indium(III) with halide and thiocyanate ions has been studied by calorimetry in N,N-dimethylformamide (dmf). Four mononuclear halogeno complexes [InX(dmf)₅]²⁺, [InX₂(dmf)₄]⁺, [InX₃(dmf)₃] and [InX₄]^–(X = Cl, Br, I) are found for ln^III, whereas only two complexes are found for Ga^III: [GaX(dmf)₅]²⁺ and [GaX₄]^–(X = Cl, Br). The absence of Ga^III di- and tri-halogeno complexes in solution has been confirmed by ⁷¹Ga NMR spectroscopy. The pronounced dominance of [GaX₄]^– is interpreted by the tendency of Ga^III to form a tetrahedron and the steric hindrance of solvent molecules. Both metal ions behave as a hard cation toward halide ligands, i.e. the chloro complexes are the most stable. The indium(III) complexes are by far more stable than the gallium(III) ones. Mixed halogeno complexes were also studied, and three complexes for the Ga—Cl—Br system and six complexes for the In—Cl—Br system were found. Their thermodynamics shows regularity reflecting the primarily ionic character of the metal–halogen coordination. Four mononuclear isothiocyanato complexes are found for Al^III, Ga^III and In^III, together with [In(NCS)₅]^2–.