Thermal Decomposition Reaction of Halogenoammineruthenium(III) Halide Complexes

Abstract

Abstract The thermal decomposition of halogenopentaammine- and cis-dmalogenotetraammineruthenium(III) complexes was studied by the techniques of thermogravimetry(TG), differential thermal analysis(DTA), and evolved gas analysis (EGA). The stoichiometry of the decomposition reaction was determined by the analysis of the reaction intermediates and products. The order of decreasing thermal stability, as well as order of temperature of massloss initiation, seems to be: [RuCl(NH3)5]Cl2>[RuBr(NH3)5]Br2>[RuI(NH3)5]I2 for monohalogeno complexes, and cis-[RuCl2(NH3)4]Cl>cis-[RuBr2(NH3)4]Br>cis-[RuI2(NH3)4]I for dihalogeno complexes. Trihalogenotriammine complexes were observed as intermediates for the cases of dichloro and bromo complexes, a simple decomposition reaction was found for the iodo complexes, and then the activation energy for the following reaction was determined: [RuI(NH3)5]I2→RuI3+5NH3, Ea=41.9 kcal/mol and cis-[RuI2(NH3)4]I→RuI3+4NH3, Ea=27.8 kcal/mol. The reaction mechanisms may be summarized as follows: (a) an electron-transfer mechanism, in which the ruthenium(III) was reduced by the ammonia molecule or the halide ion, (b) the thermal splitting of the ruthenium-ammine bond, and (c) the bond formation between the ruthenium(III) and the halide ion after the ion migration from the outersphere to the innersphere of the complex. The predominant factors which control these mechanisms may be the coordinate bond strength of the ruthenium(III) and various ligand species, the relative potential of electron transfer from the ligand to the ruthenium(III), and the ease of halide-ion migration from the outersphere to the innersphere of the complex.