Chemistry of elements niobium and tantalum. 87

Thermal decompositions of the compounds Nb3Cl8, Nb3Br8, Nb3I8, [Nb6Cl12]Cl2, [Nb6I8]I3, TaCl3, TaBr3, [Ta6Cl12]Cl3, [Ta6Br12]Br3, [Ta6Br12]Br2 and [Ta6I12]I2 have been investigated by means of the thermobalance. These observations have been completed by chemical analysis of the solid phases, X-ray photographs, extractions of the solid with ethanol and mass spectra. The formation of the crystal lattices containing Me6-complexes during the thermal decomposition of MeX3 or Me3X8 is kinetically difficult. The decomposition in the vacuum requires much higher temperatures as expected from thermodynamics. During the decomposition of TaCl3 occurs an X-ray amorphes phase, from which Ta6-complexes can be extracted. The retardation in respect to the formation of the corresponding crystal lattice is explained by the assumption that the complexes within the amorphous phase have different oxidation numbers. This requires a redox mechanism connected with diffusion. The decomposition reaction leading from [Ta6Br12]Br3 to [Ta6Br12]Br2 proceeds in such a way, that a small part of the complexes is decomposed and acts as reducing agent for the remaining undestroyed complexes. (author)