Bonding in cluster compounds

Abstract

The study of cluster compounds is an active area of current research. The scope is widening and systematic methods of synthesis beginning to appear based, for example, on the isolobal principle first mentioned in Chapter 10 and which will be met again later in this chapter. It seems likely that it will eventually be found that such compounds may involve almost any element, perhaps with the exception of the most electropositive. Some representative examples are given in Fig. 15.1. Despite all this activity, our understanding of the bonding in cluster compounds is relatively primitive. Most of them are beyond the scope of detailed ab initio calculations and resource has to be made to more approximate methods, the Xα and its refinements probably being the most reliable (see Section 10.3.1). However, it seems that even the simplest of methods, in particular the extended Hückel (Section 10.5), can give the highest occupied orbitals with a tolerable accuracy, though being far less reliable for more deeply lying orbitals. Unfortunately, cluster molecules have such a plethora of bonding molecular orbitals that any photoelectron spectroscopic data (Section 12.7) can only be interpreted with the greatest difficulty. So, it is difficult to find any reliable external check on the available calculations.