Electron Counting Rules for Gold Clusters Which Are Stereochemically Non-rigid and Exhibit Skeletal Isomerism

Abstract

The valence shell electron pair repulsion theory (VSEPR) added a three-dimensional interpretation of simple molecular inorganic compounds by proposing that bonds and lone pairs are both stereochemically active and together define a characteristic co-ordination polyhedron. The polyhedral skeletal electron pair theory (PSEPT) developed an analogous relationship for borane and metal carbonyl polyhedral molecules which is based on the total number of skeletal valence electrons in related closo-, nido- and arachno- deltahedral molecules. The historical development of the ideas which led to the PSEPT has been described in the Introductory Chapter of Volume 1. The electron counting rules associated with this generalisation have proved to be useful as aide-memoire for teaching inorganic chemistry since they bring together a large body of structural information for polyhedral molecules. Their simplicity provided an easily applicable, but reasonably reliable, indicator for identifying potential new targets for synthetic chemists. The fundamental question remains – “What do you do if the rules do not work?”. This chapter provides an account of the steps taken when a significant group of cluster compounds were found not to comply with the rules. The gold clusters discussed in this chapter were recognised as such a group in the 1970s, but they did not follow the pattern which had been recognised earlier for polyhedral boranes and transition metal carbonyl clusters. Furthermore, as their structures were revealed by X-ray crystallography it became apparent that they provided many examples of skeletal isomerism, i.e. more than one skeletal geometry was observed for seemingly isoelectronic molecules. In addition, spectroscopic studies showed that many of the clusters were stereochemically non-rigid especially in solution. The challenge of bringing these gold clusters within the scope of PSEPT when they do not share the same structures and electron counts as metal carbonyl clusters have multiple structures and are frequently stereochemically non-rigid was not trivial. Most importantly it was not sufficient to just tinker with the numerological aspects of PSEPT but a re-examination of the fundamental quantum mechanical assumptions underlying PSEPT and reformulating it in the new context.