Relative acceptor strengths of the gallium trihalides towards pyridine and triethylamine as donors

Abstract

The 1 : 1 adducts of triethylamine with gallium trichloride, tribromide, and tri-iodide have been prepared and characterised by melting point–composition diagrams. Two other new compounds C₅H₅N,Gal₃ and 2C₅H₅N,GaBr₃ have also been prepared.

The heats of formation of the 1 : 1 pyridine adducts of gallium tribromide and tri-iodide and of the 2 : 1 adduct with the tribromide were determined calorimetrically and compared with the corresponding data for the pyridine adducts of gallium trichloride. The results have been used to estimate the gas-phase heats of formation, the intrinsic heats of addition, and the donor–acceptor bond dissociation energies; each of these quantities follows the order GaCl₃ > GaBr₃ > Gal₃. Comparison with the corresponding pyridine adducts of the boron and aluminium trihalides indicates that the relative acceptor strength towards pyridine follows the order AlX₃ > BX₃ > GaX₃.

The heats of formation of the triethylamine adducts of the gallium trihalides were also measured; there is a dramatic decrease in acceptor strength in the sequence GaCl₃ GaBr₃ > Gal₃. This is interpreted in terms of a steric effect.