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A systematic TG/DTG/DTA analysis is reported of anhydrous and heptahydrate forms of tris-phenanthroline and tris-pyridyl complexes of nickel(II), whose kinetic parameters were calculated by five different methods. The dehydration and de-ligation steps are descrete in Ni(phen)3Cl2 9 7 H20, while those in Ni(bipy)3Clz " 7 H20 are mixed. Partial loss of the ligand is common for both hydrated and anhydrous compounds. In most cases the activation energy E a calculated from the mechanism-noninvoking equation of Horowitz and Metzger is in good agreement with that found from the mechanism-based relation of Mampel. There is an appreciable variation in magnitude in the results obtained from the different relations for the evaluation orE a. It is inferred that such kinetic data are of significance in comparisons of the decomposition processes in related systems but not as absolute quantities.In spite of the large number of studies reported on the thermal analysis of coordination compounds, there are relatively few reports [1-4] on the calculation of the kinetic parameters and their relationship with the nature of the bonding. This may be due primarily to the uncertainties involved in the experimental measurements. Sestak et al. [5] suggested two mathematical approaches, a mechanism-noninvoking one and mechanism-invoking one, for studying the kinetics of decomposition from dynamic TG experiments. Both approaches have previously been applied only to isothermal [6] weight-change studies. Nair and Madhusudanan [4] have recently extended these methods to the deamination of metal complexes. In the present paper we report studies on the kinetics and mechanism of thermaldecomposition of anhydrous and heptahydrate nickel complexes: Ni(bipy)3C12, Ni(phen)3C12, Ni(bipy)3Cl2 " 7 H20 and Ni(phen)3CI 2 9 7 H20. Thermal decomposition studies have been reported on similar compounds: Ni(bipy)3Br2 " 5H20 and Ni(bipy)3Br2 [7]. Calculations of the activation energy, Er, and the order of the reaction n, are carried out using data from TG, DTG and DTA curves on the basis of the standard treatments reported in the literature. ExperimentalSynthesis of the complexes -Unless otherwise stated, all the reagents used were Fluka products of reagent grade, and were used without further purification.

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Thermogravimetric analysis, TGA, of cobalt(II)bromide, [Co(NH3)6]Br3 –henceforth referred to by the shortname HCBR– was carried out using a wide diameter short one and a narrow diameter long sample cups. In the former type of cup the TGA trace was similar to that reported one. However, when the latter cup was used, the TGA showed oscillations in the % weight-loss. Cobalt(II)bromide, CoBr2, is formed as an intermediate product in the thermal decomposition of HCBR. The oscillatory nature of weight-loss curve was studied for both HCBR and CoBr2. The oscillatory behavior in the present system has been confirmed on the basis of the following experiments: (a) Dynamic TGA and differential thermal analysis, DTA, of HCBR and CoBr2·2H2O. (b) Isothermal TGA by weight-loss method for HCBR and CoBr2. (c) Thermal decomposition studies, in closed system, of (i) CoBr2 and of (ii) CoO in a mixture of O2 and Br2 gases, and of a mixture of (iii) Co3O4 and CoBr2 in dry nitrogen. The oscillations in the composition of the solid residue were attributed to the formation of CoOBr as an intermediate. Final product of thermal decomposition of CoBr2 in air is Co3O4. Hence it is possible to imagine that the point of CoOBr formation makes branching off point. This sets the reaction to oscillate in the composition domain. Therefore, it was thought that Edelstein mechanism would be operative in this reaction. Alternately, the population growth model may also be used to explain this observation. Formation of CoOBr was thought to be the weight-loss step, i.e. decrease in the ‘population’ of CoBr2, while the formation of CoBr2 was the weight-gain step, i.e. increase in the ‘population’ of CoBr2 under the proposed reaction, CoBr2 + 1/2O2   CoOBr. The purpose of this article is, therefore, to present the interesting facts of this reaction before the readers, and not to ascribe any particular model for its explanation.