Metallocarboranes bonding

Formation of group-IIIB-group-IB and -IIB element bonds is limited almost exclusively to the element B and is observed predominantly in metalloboranes and metallocarboranes. The boron-IB and boron-group-IIB bonding interactions either are of the a type or can be described as multicenter bonds ... 

Carboranes also offer the capacity for Co111—C bond formation. Perhaps the best-known example of a mixed cyclopentadienyl-metallocarborane sandwich complex is the cobalt(III) compound (Cp)Co(l,2-C2B9Hn), which serves as a precursor for other species.544... 

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... 

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. 

Metallocarbenoids, in addition reactions, for C-C bond formation, characteristics, 10, 320 Metallocarboranes, with Zr(IV), 4, 982... 

In this review, we treat in depth the synthesis, structures, properties, and reactions of -bonded metallocarboranes. Our survey is restricted to complexes of 2-carbon carboranes and to species that have between nine and fourteen total polyhedral vertices. Coverage of metal complexes of other heteroboranes is available in Grimes s book (41) and in Todd s review (93). The recent work of Grimes and his group has concentrated on metallocarboranes having fewer than nine vertices (42, 75, 76). 

Boranes, carboranes, and metallocarboranes are electron-deficient compounds in which B-H-B three-center two-electron (3c-2e) bridged bonds are found. The B-H-B bond results from the overlap of two boron sp3 hybrid orbitals and the Is orbital of the hydrogen atom, which will be discussed in Chapter 13. 

Sections II-X are not exhaustive. For example, they contain no reference to metallocarboranes or to macrocyclic species containing metal—carbon bonds, such as the vitamin B12 models (32). However, we have attempted to include material which relates electrochemistry to other fields. For example, we note the formation of electron-transfer products by 60Co y irradiation, negative ion mass spectrometry, and photoelectron spectroscopy. We also describe pairs of complexes apparently related by one-electron transfer but which have not been studied electrochemically. Clearly, such studies would provide confirmatory evidence for the proposed formulations. 

Metallocarboranes interact with phosphines to form various types of complex in which the phosphorus atoms are bonded to the carbon atoms. One example is provided by the lithium derivative of BiqHio 2CH, which can give compounds according to scheme (9.117). These products include metal-coordination complexes with metal carbonyls and highly polymeric materials with molecular weights up to 10,000. The structure of the dimer chloride in Equation 9.117 is indicated in Figure 9.5. 

In sandwich compounds and metallocarboranes an alternative distortion mode is observed which converts an antibonding orbital into a non bonding orbital. This distortion has been described as a slip distortion [42-43] and involves the mutual displacement of the rings or nido polyhedra shown in (9). The manner in which the antibonding... 

Emphasis in this report is placed on chemistry that involves the aromatic ligand ultimately. Reactions that involve replacement or elaboration of other ligands present are generally not included. Similarly, a very selective approach is taken to Cp or arene complexes that contain metal-metal bonds, carbene or carbyne, or hydrocarbyl ligands (see Chapters 9-13). Only those metalloborane and metallocarborane complexes that incorporate a Cp or arene ligand are described and crystal structure determinations are included only where results of particular significance or solutions to structural problems are provided. 

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