A Pictorial Approach to Molecular Bonding

With the development of accurate molecular calculations in recent years, useful predictions of molecular electronic properties are currently being made. It is therefore becoming increasingly important for the non-theoretically oriented chemist to appreciate the underlying principles governing molecular orbital formation and to distinguish them from the quantitative details as­ sociated with particular molecules. It seems highly desirable then that the non­ theoretician be able to deduce results of general validity without esoteric mathematics. In this context, pictorial reasoning is particularly useful. Such an approach is virtually indispensable if bonding concepts are to be taught to chemistry students early in their careers. Undergraduate chemistry majors typically find it difficult to formulate molecular orbital schemes, especially delocalized ones, for molecules more complicated than diatomics. The major reason for this regrettable situation is the general impracticability of teaching group theory before students take organic and inorganic courses, wherein the applications of these concepts are most beneficial. Consequently many students graduate with the misconcep­ tion that the ground rules governing bonding in molecules such as NH3 are somehow different from those which apply to aromatic systems such as C H • 6 6 Conversely, seniors and many graduate students are usually only vaguely, if at all, aware that sigma bonding (like extended pi bonding) can profitably be described in a delocalized manner when discussing the UV-photoelectron spectrum of CH , for example.