Carborane anions, metal complexes

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 ... 

The controlled synthesis of carbons adjacent nido- and arachno-carborane anions can be achieved by the reduction of C (cage)-C (cage)-linked o-carboranes with group 1 metals.234-237 Typical examples are illustrated in Scheme 22. These have been found to undergo metallation reactions similar to their carbons apart analogs, forming full- and half-sandwich complexes of lanthanide metals.238... 

The stability of the liquid carboranes depends on the substituents R at carbon and boron. The axial (endo) hydrogen atom is acidic and involved in 3c2e bonding to one of the basal boron atoms. In the 1H NMR it exhibits a high field shift near S = —1.4. Deprotonation of 55 with potassium or Bu Li leads to the anion (55-H)-, which is isolobal with C5H5. Reactions of 55 and (55-H) with appropriate metal complexes lead to metallacarboranes with sandwich structures [67, 69],... 

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. 

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 QB, open face and forming a single isomer. Other preparative routes, such as metal insertion into neutral carboranes at high temperature, involve more complex processes and tend to give less clean-cut results, and such approaches are rarely employed today. Typical syntheses that are based on readily available 11-vertex and 7-vertex nido-carborane ligands are as shown ... 

Cyclic organoboranes containing mnltiple boron atoms within the ring stracture have been extensively stndied as precursors to (multidecker) transition-metal complexes and to nonclassical organoboranes (see Boron Metallacarbabo-ranes and Boron Polyhedral Carboranes). Earlier efforts have been thoroughly reviewed. Recently, a series of triboracyclopentane derivatives were reported by Bemdt and Schleyer (Scheme 6). " Reduction of these boracy-cles gave the nonclassical bishomoaromatic anions (28) and homoaromatic dianions (29). Related trishomoaromatic dianions obtained via reduction of 1,3,5-triboracyclohexane derivatives with lithium were studied by Siebert and Schleyer. i 5> ... 

BioHioCNH2R, and BioHioCH has been reported. The syntheses are based on two general reactions very closely related to the preparative methods recently described for the [3]-l,2-dicarbaundecahydroundecaboranyl [ (3)-l,2-dicarbollyl ] sandwich complexes (1) the reaction of NasBioHioCH and an anhydrous metal chloride, and (2) the reaction of CsBioHisCH, aqueous sodium hydroxide, and a metal chloride. Complexes of Fe(III), Co(III), and Ni(IV) with the three types of monocarbon carborane anions indicated above can be conveniently prepared by the aqueous route. The preparations of CssNi-(BioHioCH)2 and (BioHioCNH3)2Ni by this route are described here. 

Another series of carborane anion complexes of transition metals is formed from the B7C2H ion.32 For example, when the B7C2Hjf ion mentioned in Section 8-11 is treated with cobalt(n) chloride, the following reaction occurs 2B7C2H r + 1.5Co2+ - t(B7C2H9)2Co]- +0.5Co + 2H2 32 T. A. George and M. F. Hawthorne, J. Amer. Chem. Soc., 1969, 91, 5475. 

Besides transition metal complexes with weakly coordinating cteo-borate anions, metallacarboranes in which metal atom incorporated into carborane cage are of great interest... 

Various anionic derivatives of lower carboranes, containing from six to nine cage atoms, have been prepared and found to form stable transition metal complexes, e.g. 

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