A comparative study of the thermal stability of coordination compounds by thermoanalytical methods

Abstract

The informative capacity of thermal analysis in the evaluation of the thermal stability of coordination compounds involving the evolution of volatile ligands is discussed. The temperature of decomposition under quasi-equilibrium conditions (Q-derivatograph, quasi-isobaric, quasi-isothermal operation) is suitable for characterizing the thermodynamic stability of compounds. The initial temperature of decompbsition at linear heating indicates the reaching of a defined value of the rate constant (depending on the sensitivity of the sensing device of the instrument and on expel'imental conditions). The temperature sequence of increasing thermal stability may not coincide with the sequence of increasing activation energy values, since in the majority of cases it demands on the value of activation enthropy.Many thermal reactions of coordination compounds involving the evolution of volatile ligands may be regarded as heterogeneous ligand exchange processes. It is therefore of interest to compare thermodynamic and kinetic stabilities for series of coordination compounds in similar-type. The sequences may be obtained if the equilibrium constants and the rate constants of the reactions in question are known at some standard temperature. It is difficult, however, to choose the standard temperature. Known sequences of stability for reactions of coordination compounds in solution are based, for instance, on data of stability constants determined at 25 ~ , and the total temperature range of the experiment does not exceed 0 to 100 ~ (at atmospheric pressure). The majority of crystalline coordination compounds are being synthetized at ambient temperature, and are rather stable at 25 ~ both kinetically and thermodynamically. The temperature range of interest for studying solid-phase transformations is much broader.Since thermal analysis is a traditional method for studying coordination compounds efforts have been made for a long time past to obtain qualitative sequences of their thermal stability indirectly from thermoanalytical data without determining the numerical values of the equilibrium constants and rate constants (at standard temperature).The sequences of the thermodynamic stability of isostructural compounds in similar-type can be obtained using the Q-derivatograph [1-2]. The particular features of the experimental method are as follows: dynamic heating of the sample at a determined rate takes place only up to the start of weight loss. As soon as weight loss begins a special system takes over the control of oven heating, the 5

The thermal decompositions of some transition metal acrylates and polyacrylates

“…However, the sequence of increasing E values for all the investigated compounds does not correlate with their thermal stability sequence. The temperature sequence of increasing thermal stability may not coincide with the sequence of increasing activation energy values, since it can also depend on the value of the activation entropy [19]. The lack of such a correlation has been observed in the dehydration reactions of EDTA chelates of a number of metal ions [19], and also in the dehydration and decomposition reactions of mixtures of zinc and copper oxblates [20].…”

The relationship between the structures of Cu(II) complexes and their chemical transformations

“…The formation of this compound as an intermediate of thermal decomposition of CuSO~ 9 5 H20 was doubted for a long time. The decomposition temperature is interpreted as the temperature at which a given experimental apparatus allows the decomposition rate to be registered [5]; however, the decomposition temperature is often brought into connection too with the strengths of the bonds between the central atom and the ligands [17]. Qualitative experiments showed, however, that this compound, differently from CuSO4 " 3 H20, was no longer hygroscopic.…”

Errors in the evaluation of the thermal (kinetic) stability of compounds by means of the start temperature of thermal decomposition in the presence of isokinetic points

“…If one single topochemical mechanism is valid for the whole series of compounds studied, this will result in identical rate constants of thermal decomposition at the temperatures at which decomposition starts. In this case (if certain special requirements to experimental conditions are satisfied) the temperature sequence may coincide with the traditional sequence of kinetic stability (relative to the rate constants at a standard temperature) [1 ].Itowever, the possibility that isokinetic points exist cause difficulties in the analysis of kinetic stability.Let us consider, by way of example, the thermal decomposition of urea (U) inclusion compounds with n-alkanes. The thermal decomposition kinetics of the compounds were investigated under non-isothermal conditions, using a continuousflow reactor (sample mass 5... 10 mg, rate of temperature increase 4... 6~ flow rate of helium 140 cm3/min) [2].…”