Carboranes From Small Organoboranes to Clusters

Armin Berndt, Matthias Hofmann, Walter Siebert and Bernd Wrackmeyer 

This chapter summarizes recent developments in the expanding field of electron-deficient compounds having from three up to 13 skeletal boron and carbon atoms. In particular, the focus will be on the transition of classical organoboranes into non-classical compounds. Therefore, we first want to briefly review electron counting rules and bonding characteristics of these classes. For a more thorough discussion see Chapter 1 by King and Schleyer. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all 

In order to define a borderline between classical and non-classical structures we introduce the following criteria compounds are classified as non-dassical if their framework atoms employ multicenter cr, or multicenter cr and n, or n,cr-distorted multicenter bonding to cope with electron deficiency. 

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Berndt, A. Hofmann, M. Siebert, W. Wrackmeyer, B. Carboranes From Small Organoboranes to Clusters. In Molecular Clusters of the Main Group Elements-, Driess, M., Noth, H., Eds, Wiley-VCH Weinheim, 2004 pp 267-309. 

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