Boron hydrides arachno

Figure 3-19. Boron skeletons of boron hydrides. Left Regular icosahedral boron skeleton of B12H122 Right Closo, nido, and arachno boranes after Williams [38] and Rudolph [39], The genetic relationships are indicated by diagonal lines. Used with permission, copyright (1976) American Chemical Society.

Statement (c) is only partially correct— while Lewis acidity of BX3 does increase from F to Br, it is not due to stronger B-Br n bonding (as stated) but exactly due to the opposite trend the tc bonding is decreasing in the same order Lewis acidity is increasing, that is, n bonding in B-Br is weaker than in B-F (the latter explains the former). Statement (d) is incorrect—arachno boron hydrides are less stable than nic/o-borohydrides, also, their skeletal electron count is n+3 not 2(n-t-3). 

A nido (nestlike) boron hydride is derived from a closo borane by the removal of one skeleton atom. If the starting closo borane is not a regular polyhedron, then the atom removed is the one at a vertex with the highest connectivity. An arachno (weblike) boron hydride is derived from a closo... 

There are various types of boron hydride and borohydride ion. Those with a closed polyhedron of boron atoms are designated closo compounds. Ones in which there is an incomplete polyhedron (by removal of one vertex) are called nido compounds (from the Greek word for nest ). Those with a more open structure (removal of two or more vertices) are arachno compounds (from the Greek spider ). 

FIGURE 10 The structures of two nido (left) and one arachno carboranes. The small filled circles represent hydrogen atoms. [From Onak, T (1975). In Boron Hydride Chemistry (E. L. Muetterties, ed.), p. 353, Academic Press, New York, Figures 10.3 and 10.4.]... 

The closo boron hydride anions have been studied considerably more than the open ions, nido and arachno. The reactions of the open ions are quite varied, and we will consider only a few of these before proceeding to the reactions of the closo anions. 

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). 

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