Electronic configuration and bond energy

Conclusions

The original interpretation of the method of molecular orbitals as a theory of valency, in which the single electron possesses bonding power was based on the assumption that non-premoted electrons are bonding and premoted ones are anti-bonding or non-bonding. This assumption is not the outcome of any requirements of theory but is an empirical postulate, which seeks justification in experimental facts. Earlier attempts at correlation between the electronic levels of molecules and the energy states of the constituent atoms indeed appeared to lend support to this assumption. Later investigations have, however, shown that completely different correlations are possible and are absolutely necessary to satisfy the experimental facts. For example, in a molecule like BeO the stable triplet term, which should arise from unexcited atoms according to the older correlation, is not found and by the correlation of certain excited terms to anomalous terms of the metal atom it is shown, that a non-premoted odd electron in the configuration of the ground level lowers its dissociation energy. Similar remarks apply to the types BeF and NO and the new correlation of molecular terms to atomic states from the pair-bond view is shown to be satisfactory without exception in a larger number of molecules of these three classes, recently.

These three types represent molecules with free valencies and indeed just in such molecules the difference of the two view-points must become apparent. For other such molecules with free valencies, whose band spectrum is investigated in detail,i.e., for the oxides of the third and the halides of the fourth group of the periodic system, only one example of each,i.e., AlO and SnCl, was originally available. We have now shown in the present paper, that the correlation according to the pair-bond theory of valency holds also for other molecules of this type and appears to be generally valid.All this evidence, concerning the band spectra of oxides and halides of the second, third, fourth, and fifth groups of the periodic systemis definitely against the postulate of the identification of premoted and non- premoted with non-bonding and bonding electrons. On the other hand, such a satisfactory correlation from the pair-bond view necessitates a revision of the theory of valency, based on the results of band spectroscopy.