Metallaboranes and Metallacarboranes

There are many metalloboranes and metallocarboranes. Selected examples with closo structures are given in Table 15-10. 

FIGURE 15-18 Comparison of C2B9Hii with CsHj. (Adapted with permission from N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1984, p. 210. 1984, Pergamon Press PLC.) 

Valence Electrons in Organometallic Fragment Example Replace with 

Classify the following metallaboranes by structural type a. C2B7H9(CoCp)3 b. C2B4H6Ni(PPh3 2 

Although the comparison between these ligands is not exact, the similarity is sufficient that C2B9Hn can bond to iron to form a carborane analog of ferrocene, [Fe(i7 -C2B9Hii)2] . A mixed-ligand sandwich compound containing one 

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Metal-vapor synthesis also prepares metallaboranes and metallacarboranes with oxidation of the metal. Thermally generated Ni, Co and Fe atoms react with the nido-carborane 2,6-C2B7H, and either cyclopentadiene, toluene, mesitylene, or... 

Many boranes and carboranes can act as very effective polyhapto ligands to form metallaboranes and metallacarboranes. Metallaboranes are borane cages containing one or more metal atoms in the skeletal framework. Metallacarboranes have both metal and carbon atoms in the cage skeleton. In contrast to the metallaboranes, syntheses of metallacarboranes via low- or room-temperature metal insertion into carborane anions in solution are more controllable, usually occurring at a well-defined C2BK open face to yield a single isomer. 

A large number of metallacarboranes and polyhedral metallaboranes of s-, p-, d-, and f-block elements are known. In these compounds, the carboranes and boranes act as polyhapto ligands. The domain of metallaboranes and metallacarboranes has grown enormously and engenders a rich structural chemistry. Some new advances in the chemistry of metallacarboranes of f-block elements are described below. 

Table 13-7 Metallaboranes and Metallacarboranes with Closo Structures...

Metallaboranes and metallacarboranes can be classified structurally by a procedure similar to that for boranes and their main group derivatives.30 In this scheme, the valence electron count of the metal-containing fragment is first determined and then compared with the requirements of the 18-electron rule. This fragment can then be considered equivalent to a BH fragment needing the same number of electrons to satisfy the octet rule. For example, a 15-electron fragment... 

It is now well established experimentally that the range of deltahedral structures possible for metallaboranes and metallacarboranes extends beyond the icosahedron to include 13- and 14-vertex deltahedra,as will be outlined later (see Section 3.10). It is also known, from calculations and some experiments, that polyhedral structures with some nontriangular faces (and so not fully deltahedral) may be preferred for boranes and supraicosahedral carboranes.What structures are experimentally possible for carboranes themselves beyond the icosahedron has yet to be fully explored. We focus here on systems with up to 12 skeletal atoms. 

Although metallaborane and metallacarborane chemistry is extensive, little information is available on metal nuclear shielding in these compounds, and is essentially limited to some nice work on Be, some isolated Pt chemical shifts, " a ( Y) datum for [(thf)2YBjHi2], and a ( Zr) datum for [ZrB4Hie]. Very recent work has included 295 (193-195) 453 (188) H9sn.(i 5)... 

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