Metallocarborane complex

The polyhedral contraction route to metallocarboranes consists of the degradative removal of a polyhedral boron atom of a metallocarborane followed by oxidative closure of the resulting m do-metallocarborane complex to a closo species having one fewer vertex than present in the starting material (68) ... 

The following reaction sequence leading to these complexes is believed to be similar to the stepwise reactions in which metallocarborane complexes are formed from neutral cZoso-carboranes ... 

CpMo(CO)2 P(BH3)Ph[N(SiMe3)2 If and metal-locyclopentadienyl and metallocarborane " complexes such as 8 and 9 with agostic geometries similar to a Si analogue (Stmcture Xin in Table 11.1). 7-9... 

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. 

Cu ( j -C5H5)2] is not. Likewise, Fe and Ni carborane derivatives are extremely stable. Conversely, metallocarboranes tend to stabilize lower oxidation states of early transition elements and complexes are well established for Ti", Zr , Hf , V , Cr and Mn" these do not react with H2, N2, CO or PPh3 as do cyclopentadienyl derivatives of these elements. 

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). 

Because the complexes listed in Tables 1-8 are all prepared with metal halides and carborane anions, only one example for each type of metallocarborane (number of B atoms) is given in the following representative equations for clarity, the skeletal framework in the equations have numbered positions and H atoms are omitted ... 

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... 

Metalloboranes Their Relationships to Metal-Hydrocarbon Complexes and Ousters, 21, 57 Metallocarboranes, Ten Years of, 14, 145 Methylene Bridge, 20, 159... 

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). 

Our approach to the subject has been to divide the metallocarboranes according to the size of the polyhedron. Starting with twelve-vertex compounds, which constitute the majority of the effort, we proceed to the larger polyhedra, so far unknown in the B H 2 and C2B 2H series, and then to the lower polyhedra. Further subdivisions within each polyhedral size include synthesis, structures, and properties of monometallic complexes, reactions of monometal lies, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. 

Five major synthetic routes are now available for the preparation of metallocarboranes, although only one was well established in 1969. The recently developed synthetic methods have allowed the preparation of more complex and diverse compounds and have greatly expanded the field of metallocarborane chemistry. These preparative methods are discussed in some detail in this section, for the synthesis of all the known metallocarboranes have been accomplished by one or more of these routes. 

Although a number of different metallocarboranes have been synthesized in high yield by this reaction (18, 24), the actual chemistry is frequently much more complex than implied by Eqs. (5) and (6). For example, products containing greater and fewer numbers of boron atoms than present in the carborane starting materials have been isolated from polyhedral expansion reactions. 

The polyhedral expansion reaction appears to be a general synthetic method for metallocarboranes all the known doso-carboranes have been found to produce metal-containing compounds when subjected to the re-duction-complexation operations of this synthetic scheme. Moreover, metallocarboranes containing more than one transition metal may be prepared by the polyhedral expansion of monometallocarboranes. Examples of this synthetic route will be described in following sections. 

Spencer, Green, and Stone (88) have recently described a new synthetic approach to metallocarboranes in which an organomctallic transition metal complex is thermally reacted with a neutral cZoso-carborane, resulting in the incorporation of the metal into the polyhedron ... 

Limited investigations of the chemical reactions of metallocarboranes have been reported. In general, these complexes are much less reactive than the analogous metallocenes, yet several unique new species have been prepared from the unsubstituted metallocarboranes. 

This protonated metallocarborane is unique in that it undergoes ready substitution at polyhedral boron atoms, whereas the unprotonated species is unreactive in the absence of acid. Reaction of (1,2-C2B9Hn)2FeH- with Lewis bases such as dialkyl sulfides results in the loss of H2 and the formation of boron-substituted complexes in good yields ... 

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