Metallocarboranes and metalloboranes

The nickel compound is found in three oxidation states 

Closo cages have a p.e.c. of 4n + 2 electrons, whereas nido-carboranes require 4n + 4 electrons. The larger doso-carboranes can be reduced by alkali metals to nido anions. The latter can then often be converted into doso-metallocarboranes on treatment with a metal halide  

Metalloboranes have also been prepared in which a metal atom has been incorporated into a borane structure. While the lower boranes such as and BjHg are extremely air sensitive as well as being thermally rather unstable, the metalloboranes are often robust. Cobalt(II) chloride with a mixture of B5H and Cp affords a 5% yield of the red air-stable crystalline 2-CpCoB4Hg, which isomerizes at 200 C to the yellow 1-isomer (Fig. 11.9). The isoelectronic complex l-(0C)3FeB4Hg is one product from the reaction between Fe(CO)5 and BgH. The borane ligand in these complexes, B H, is isoelectronic with and hence forms complexes of similar structure to C H CoCp and C4H4Fe(C0)3 (p. 269). 

The polyhedral electron count usually correlates with the structure in the case of metalloboranes and metallocarboranes, as well as for the parent compounds. 

To calculate the p.e.c. the valence electrons of all the skeletal atoms are summed, together with the number of electrons contributed by the ligands (p. 192). This sum is characteristic of the structure of the cluster (Table 11.1). In order to meet cluster bonding requirements, transition metal atoms require ten more valence electrons (to fill the (n — l)d orbitals) than Main Group atoms. 

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Metallocarboranes and metalloboranes, discussed in 6.5.4, may be prepared from metal halides and carborane anions. Selected examples of complexes formed by thermal—or otherwise—induced polyhedral rearrangements of existing metallocarboranes are given in a few equations, and some other individual compounds are listed in tables. 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

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