Diazo Derivatives of Polyhedral Boron Hydrides

Inorganic diazonium salts have been reviewed by Laali and Olah (1985). Diazosilane (SiH2N2) is an inorganic analog of diazomethane. It has not yet been observed or isolated, but its structure and stability were calculated by Thomson and Glidewell (1983) using MO methods. They are discussed in the section on isomers of diazomethane (Sect. 5.4). 

All inorganic diazonium ions (X-Ni ) discussed in Section 3.1 are characterized structurally, first, by a small group X consisting of one to five atoms, and, second, by little or no stabilization of the X —N(a) bond. These inorganic diazonium ions are, therefore, comparable to simple aliphatic diazonium ions, e.g., H3C Ni.  

There are, however, also a few inorganic diazo compounds in which X has 18 or more atoms and, in addition, the members of this group have a stability and reactivity comparable to those of aromatic diazo compounds. These are the mono- and bis(diazo)boranes, which were discovered by the group of Muetterties in their work on cluster-type boron hydrides (Knoth et al., 1964). 

3 Inorganic Diazo Compounds and Metal Complexes with Dinitrogen as Ligand 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

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