Bond classification, 2-7 energies

Allen s book discusses equilibrium constants, entropy in terms of disorder, and formulae for the derivation of values of entropy from heat capacity measurements. An important part of the book discusses energy changes in relation to the Periodic classification and to chemical bonding. Gibbs energies of formation are plotted for some series of compounds (e.g. oxides and chlorides) against atomic number. There is a thermodynamic study of the problem of selection of a catalyst for the Deacon process. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

Sanderson, R. T. 1951. An interpretation of bond lengths and classification of bonds. Science 114 670-672 1976. Chemical Bonds and Bond Energy, 2nd edn. New York Academic Press. 

At a slightly deeper level, the difficulty of this approach lies in its acceptance of a transition complex in which the original classification into a and tt electrons has been broken consequently pure tt electron theory is inadequate for the prediction of energy changes, and a complete analysis must await the inclusion of the a bond modifications at the point of attack. Preliminary attempts to include such effects have invoked hyper conjugation (Muller et al., 1954 Fukui et al., 1954a) and other factors (Dewar et al., 1956), but little progress has yet been made towards a more detailed theoretical interpretation based on more complete calculations. 

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