Personal Historical Perspective on the Development of Electron Counting Rules for Boranes and Clusters and Ken Wade’s Contribution

2021 marks the 50th Anniversary of the publication by Ken Wade’s “The structural significance of the number of skeletal bonding electron-pairs in carboranes, the higher boranes and borane anions, and various transition metal carbonyl cluster compounds” in Chemical Communications. This paper played an important role in the development of cluster and polyhedral main group chemistry and through the isolobal analogy encouraged imaginative interconnections between main group, transition metal and organic chemistry. It has been cited more than 500 times and the rules which developed from this and related papers are introduced in all modern undergraduate inorganic textbooks. It is therefore appropriate to publish a set of reviews which illustrate the influence of these generalisations on modern inorganic chemistry and give those newer to the field an insight into how the ideas which Wade introduced evolved. The chapters also give a critical account of the limitations of the approach and suggest how the subject may develop in the future.