Abstract
Previous determinations of the standard heats of formation of trivalent, first row, transition metal acetylacetonates are based upon inaccurate heat of combustion data. In this thesis, new values are reported for the standard heats of formation, at 25°, of four trivalent metal acetylacetonates, M(C[5]H[7]O[2])[3] (M = Al III, Cr III, Mn III and Fe III); the maximum associated uncertainty interval is 0.2% respectively. All heat measurements (+/- 0.01 cals.) involved the use of an electrically calibrated solution calorimeter. Previously, solution calorimetry has been exclusively confined to the thermochemical study of simple chemical processes of well-authenticated stoichiometry. Although the reactions inherent in the present work failed to conform to such simplicity, apparent thermodynamic inexactitudes were conveniently eliminated by introducing a non-volatile solvent, in which all the components in the theoretical heterogeneous complex formation reaction were soluble. The prearranged thermodynamic equivalence of two homogeneous systems simplified the thermochemical cycle, from which the standard heat of formation of the complex was derived directly by successive applications of Hess's law. The formation reaction was finally referred to the ideal gas state in order to eliminate the effect of intermolecular forces. Subsequent to the estimation of the O-H bond dissociation energy in acetylacetone (enol isomer), and the electron affinity of the acetylacetonate radical, the homolytic and heterolytic bond energy parameters were calculated for each complex by presuming the equivalence of the six metal-oxygen bonds. By convention, the ground energy state was adopted as the thermodynamic reference state for all components in the hypothetical dissociation process. The characteristic 'twin-peaked' curve, obtained from a plot of the empirical heat of formation of the gaseous transition metal acetylacetonate versus the corresponding metal atomic number, has been inadequately interpreted by specific reference to the crystal field theory. Some preliminary inferences have been drawn pertaining to the 'nature' of the metal-oxygen coordinate bond.