Nido Carboranes skeletal bonding

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

One particularly interesting category of metallocarborane is that in which a single metal atom is shared between two polyhedra that have a vertex in eommon. In effect, the metal is sandwiched between two nido-carborane residues. Examples are shown in Fig. 17. For such commo compounds, the metal can be assumed to contribute three AO s to the skeletal bonding of each polyhedron, when the (n + 1) rule for closo clusters is found to be obeyed. For example, the isoelectronic... 

Some carboranes have formulae compatible with both classically bonded and nonclassically bonded structures and exhibit valence isomerism. The tet-racarbaborane Me4C4B6Et6 is one such. As prepared by dimerization of the small closo carborane Me2C2B3Et3 [Eq. (3.2)], it has a classical adamantane-type structure, with its four CMe units linked through BEt units (1). However, when heated it isomerizes to the expected nido carborane structure (2) with a skeletal structure like that of decaborane. ... 

The neutral chloride B4CLt, with four skeletal bond pairs, and tetrahedrane C4H4 or tetraphosphorus P4 and related molecules, with six skeletal bond pairs, are systems with regular tetrahedral structures. Tetrahedral molecules held together by six skeletal bond pairs can of course be accommodated in the carborane cluster systematics as nido clusters with n atoms and n -1- 2 skeletal bond pairs, if a low connectivity (axial) vertex of the parent trigonal bipyramid is left vacant, instead of the expected high-connectivity (equatorial) vertex. 

A third approach notes the relationship between the anion [(q -CsHs) Fe(q5-C2B9Hii)]2 and the parent carborane from which it is derived, C2B10H12. An anionic [(q5-C5H5)Fe] unit in the former replaces a neutral BH unit in the latter. The capacities of these two units, [(CsHsjFe] and BH, to participate in cluster bonding are similar. Each can function as a source of two electrons, and three AOs, for use in skeletal bonding. Not only are the numbers of their frontier orbitals the same, but so are their lobal characteristics they are isolobal (Fig. 3.21).The metal unit can effectively supply a radially oriented Pzdz. hybrid orbital and two pd p.xL i,P-,d., ) hybrid orbitals to bond to the nido... 

To underline the structural and bonding relationship between these mixed metal-carbon cluster species and carboranes, we list the formulae of representative examples in Table 4.1, classified according to the numbers of skeletal bonding electrons they contain and thus according to their structural type (closo, nido, or arachno ). 

A second type of borane clusters are those with the general formula B H +4. These are classified as nido bo-ranes. Isoelectronic carboranes of the general formula C cB H +4 are produced by replacing a BH unit with a C. Nido refers to the nest-like structure of these compounds. Such a structure is obtained by the removal of one vertex from a closo structure (Fig. 22). Compounds of the general formula, B H +4 will have n electron pairs (one from each BH unit) plus four electrons or two pairs from the four additional hydrogen atoms. The total of n- -2 electron pairs will be used for skeletal bonding. Generally, any cluster compound with n -I- 2 skeletal electron pairs will adopt a nido structure. 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

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). 

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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