The Problem of Defining and Describing a Bond Ng

It is similarly problematic to define bonding on the basis of measured or calculated atomic distances. In order to identify a certain interatomic distance as typical of covalent bonding, suitable reference distances have to be found. Normally, this is not very difficult in cases where there is no doubt about the nature of bonding. However, for nonclassical structures or in the case of exceptional long bonds or bonds between unusual atoms the problem of the reference bond cannot be solved in an unique way. The same difficulties arise when other molecular properties are used to define and to describe chemical bonding [17]. 

Theoreticians often resort to an analysis of the molecular orbitals. However, orbitals are not observable entities, they have only a mathematical meaning. Furthermore, they can be transformed by any unitary transformation into a new set of orbitals without changing energy or other properties of the molecule. For example, they can be presented as delocalized or localized MOs. A unique definition of chemical bonding, however, should be independent of the form of the MOs. The chemical bond should preferentially be described with the help of a molecular quantity that is observable. 

However, the electrostatic force description is flawed in several ways. Electrons should prefer those locations in the molecule where the potential energy is most negative and electron-nuclear attraction is a maximum. Hence, they should be found in the vicinity of the nuclei rather than the internuclear region. Nevertheless, those electrons that are responsible for chemical bonding prefer 

In the last step, we have complemented the information gained in the first steps by an analysis of the Laplace field p(r) associated with p(r). It has been shown that the Laplaceian of p(r) indicates where electrons concentrate in the molecule which is useful for a description of electronic structure and chemical bonding [20]. 

This procedure has been very successful when used to describe compounds containing noble gas elements [4, 5,7,13,15]. Therefore, we will briefly outline some of the details of the three steps of describing chemical bonding. 

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