Cluster compounds carboranes

Many novel cluster compounds have now been prepared in this way, including mixed metal clusters. Further routes involve the oxidative fusion of dicarbon metallacarborane anions to give dimetal tetracarbon clusters such as (103) and (104) O (jjg insertion of isonitriles into inetallaborane clusters to give monocarbon meiallacarboranes such as (105) and the reaction of small ii/t/o-carboranes with alane adducts such as Et3NAlH3 to give the commo species (106) ... 

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric [nirio-(y HjJ2- ions and the well-known cyclopentadienide ion (Cp ) [CydJ- (Fig. 16). The isomeric nido-C2B9H1 J2- ions, which are commonly known as dicarbollide ions, and many other carborane anions, form stable complexes with most of the metallic elements. Indeed nearly all metals can be combined with polyborane hydride clusters to produce an apparently limitless variety of cluster compounds. 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

Boron Hydrides Boron Inorganic Chemistry Boron Organoboranes Boron Polyhedral Carboranes Cluster Compounds Inorganometalhc Compounds Containing Transition Metal Main Group Elements Electronic Structure of Main-group Compounds. 

The boranes, carboranes, and related compounds are also of interest in the field of cluster chemistry, the chemistry of compounds containing metal-metal bonds. The bonding in these compounds will be discussed and compared with the bonding in transition metal cluster compounds in Chapter 15. 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

There are some interesting observations too concerning the structures of polyhedral molecules. Very often they are electron deficient in the sense that there are fewer than two electrons for each close contact. The heavy atoms forming the skeleton of the molecule may be either main group atoms (for example in the boranes and carboranes) or transition metal atoms (metal cluster compounds) or both (metallocarboranes). 50 shows the structures expected from Wade s rules for five atom polyhedral molecules with six, seven and eight pairs of skeletal electrons. There are a total of fifteen skeletal orbitals... 

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. 

Such cluster compounds divide naturally into four major types, (a) Main group compounds (22.1-22.3). These include the boranes," carboranes, and polyhedral... 

Wade, K. (1971) The stmctural significance of the number of skeletal bonding electron-pairs in carboranes, the higher boranes and borane anions, and various transition-metal carbonyl cluster compounds. J. Chem. Soc., D Chem. Commun., (15), 792-793. 

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. 

When one or more of the vertices of a carborane are replaced by metal (M) atoms or metaUo (ML ) groups, a metaUocarborane is formed. Transition metal atoms in these cluster compounds are considered to be pseudo-octahedrally coordinated with valence shells having 18 electrons, corresponding to noble gas configuration. When this assumption is combined with Wades 2 + 2 rules, we can readily arrive at a general formula for the number of valence electrons C for the skeleton of a metaUocarborane ... 

Structure-Activity Analysis of Selected Carboranes Single-Cluster Compounds...46... 

STRUCTURE-ACTIVITY ANALYSIS OF SELECTED CARBORANES SINGLE-CLUSTER COMPOUNDS... 

Generally, the oxonium ring in carborane-dioxane 1 is opened by nucleophilic attack of V-nucleophile giving rise to a zwitterionic V-nucleophile 2. This intermediate can be reacted again with carborane-dioxanate 1 providing double-cluster compounds 3 (e.g., GB-80). If a boron building block with primary amine function is used as the ring opener, three-cluster molecules can be... 

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