Structure metallacarborane

Structurally, metallacarborane chemistry is the most fully documented area of carborane chemistry. There is not enough space here for a comprehensive survey, but Tables 3.2 and 3.3 and Figure 3.22 show the formulae and skeletal structures of some representative examples. More comprehensive compilations of structural data are to be found in References 1-3 and 199. Metals have been incorporated into the whole range of known carborane deltahedra, and indeed their presence has allowed supraicosahedral systems to be made that are without precedent among metal-free carboranes or boranes, as outlined below. 

Chemistry and structure of semi-sandwich platinum metal metallacarboranes derived from nido-Ci39Hi2 97JOM(536/537)51. 

Our discussion will be restricted to metallacarboranes in which the r/-block metal is incorporated into the polyhedral framework of the carborane. No attempt will be made to cover those compounds where the transition metal atom is in a bridging group linking several carborane polyhedra together or is involved solely as a member of a substituent group. Since much of the insight into the chemistry of the metallacarboranes is obtained from structural and bonding... 

Figure 6 Crystal structure of the mixed ligand neutral metallacarboranes of titanium(iii). Reproduced by permission of the American Chemical Society from Organometallics 1997, 16, 1365.

M = Mo, W n = 2—, 1 —, 0),54 that were discussed earlier, are consistent with these observations. Thus, several possible interactions can lead to the continuum closo—> slipped closo—> semipseudocloso—> pseudocloso—> hypercloso-structures. Hopefully, all future metallacarboranes will fall within this spectrum lest we run out of descriptive prefixes. 

A number of the heavier group 10 metallacarboranes have been synthesized and many structurally characterized. As part of their investigations of the metal complexes of the monocarbaboranes, Stone and co-workers studied the reactions of the hydrido complexes [PtH(PEt3)2(T]S-7-CB10H11)] with [AuCl(PPh3)], [HgCIPh], and [CuCl(PPh3)]4 to... 

It is apparent from the above discussions that recent synthetic, structural, bonding, and reactivity studies have established transition metal containing metallacarborane complexes as an important area of study in organometallic chemistry. 

The synthesis of new materials having novel electronic or optical properties is of great current interest. One approach to the synthesis of such materials is to systematically link small sandwich units together to form an extended multi-decker sandwich system. Immediately following the discovery of the triple-decker structure of the Cp3Ni2+ complex in early 1970s,181 Grimes and co-workers synthesized the first neutral triple-decker metallacarborane sandwich compound. Most of the subsequent research in this field, some of which has been described... 

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],... 

The first metallacarboranes were isolated in M. F. Hawthorne s laboratory and contained a metal ion, two carbon and nine boron atoms forming an icosahedral C/050-MC2B9 cage structure.It was immediately recognized that these species may be viewed as having metal ions coordinated in a pentahapto manner by the open face of a [nido-1 dianion. This was a useful formalism since it empha-... 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

Application of the polyhedral expansion methodology to C2B10H12 leads to supraicosahedral metallacarboranes such as closo-(t C HB) Co C2B1 -H 12 [33340-90-8] (194—199). Further expansion of 13-vertex species or thermal metal transfer reactions leads to the 14-vertex cluster [(T C H )Co]2C2B1 ( H 12 [52649-56-6] and [52649-57-7] (199). Similar 14-vertex species have been obtained from tetracarbaboranes (203) and show unusual structures. The isomeric bimetallic cobaltacarborane complexes c/oso-(r 3 -CpCo)2C2BgH10 (cp = C H ) can be formed by either polyhedral expansion or contraction reactions. Six isomers of this cluster are formed in the thermally-induced intermolecular metal transfer and polyhedral expansion of the 11-vertex Ao-(t b-CbHb)CoC2B8H10. 

Fig. 21. Molecular structure of the metallacarborane pinwheel cupracarborane complex [Cu3( i-H)3 C2B9H9(4-(C5H4N)COOCH3) 3] where within the cage...

Many structures are possible for the smaller metallacarboranes and various synthetic strategies are available. Especially noteworthy is the occurrence of triple- and tetradecker sandwich compounds (214). The polyhedral expansion synthetic strategy can also be used with small carboranes (212). For instance, the small metallacarborane f/ojo-l,l,l-(CO)3-l,2,3-FeC2B4H6 [3276140-3] (212) is obtained from C2B4Hg upon treatment with Fe(CO)5. 

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