Beech, Graham (1967) The thermochemistry of some transition metal complexes. Doctoral thesis, Keele University.

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The heats of decomposition of some transition metal complexes have been measured by (a) adiabatic reaction calorimetry and (b) differential scanning calorimetry. Method (a) was applied to dichlorobis(triphenylphosphine) cobalt (II) chloride while method (b) was applied to a considerable number of complexes between divalent metal halides and organic bases. These bases were pyridine, methyl-pyridines, aniline, pyrazine, methylpyrazines and pyrimidine.
The thermal data which were obtained yielded useful information which may be summarised as follows:
(i) Steric interactions The effect on the strength of the metal-nitrogen bond of methyl substitution was investigated. It was found that, for the methyl pyridines, a-substitution was effective in weakening the bond strength. It is, however, not yet understood why B-substitution appears to weaken the bond still further. Very little weakening, on methyl substitution, could be observed for the pyrazine complexes.
(ii) Crystal field effects
Certain thermal quantities, related to the heats of gas phase reactions, have been calculated from the experimental data. These, when plotted against atomic number for a particular type of complex, vary in the same manner as the crystal field stabilisation energy. One such plot was used to calculate the stabilisation energy of dichlorodipyridinenickel(II) and gave good agreement with a spectroscopic measurement of the same quantity.
(iii) Polarisability
The experimental data appear to support arguments based on the polarisability of the halogen ligand and the effect of this on metalnitrogen bond strength. n-bonding effects are proposed to account for the apparently constant metal-nitrogen bond strengths in the compounds which contain octahedral molecules. In connection with this, it is shown that there is a convergence of the heats of formation of octahedral and tetrahedral compounds after correction for crystal field effects. Since many of the decompositions were carried out at high temperatures (up to 500oC) it was necessary to calculate the magnitude of the temperature correction to the heats of decomposition required to refer them all to a common temperature. This required values for the heat capacities of the complexes and these were measured, for representative complexes, in the differential scanning calorimeter. The heat capacity data indicated, also, a rigidity in metal-pyrazine bonds which may be due to back n-donation from the metal to the ring.

Item Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
Divisions: Faculty of Natural Sciences > School of Chemical and Physical Sciences
Depositing User: Lisa Bailey
Date Deposited: 15 May 2018 14:40
Last Modified: 15 May 2018 14:40

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