General View of Bonding and Structural Relationships

The main geometries encountered in this review are shown in Fig. 1. In the closed (closo) structure (Fig. lb) the skeletal atoms occupy all corners of a polyhedron, while in the open (nido) structure (Fig. la) one corner of the polyhedron is vacant. This open face is usually the location of attachment when the carborane bonds to a metal group. In some cases, the metal can occupy common vertices of two polyhedra to give a commo structure (Fig. lc). 

H12 and [C2B9Hn]2- (or C2B9H13) both have 13 electron pairs, C2B1()H12 will have a closo structure (13 =12 + 1) and C2B9HJ3 will have a nido structure (13 = 11 + 2). 

Comparison of j5-bonding capabilities of carborane ligands and cyclopenta-dienide anion. (Reprinted with permission from R. N. Grimes (6b), in Molecular Structure and Energetics (J. F. Liebman, A. Greenberg, and R. E. Williams, eds.), Copyright 1988, VCH, New York.) 

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