Metallocarboranes

There are now at least a dozen synthetic routes to metallocarboranes including (i) coordination using nido-carborane anions as ligands, (ii) polyhedral expansion reactions, (iii) polyhedral contraction reactions, (iv) polyhedral subrogation and (v) thermal metal transfer reactions. These first five routes were all devised by 

Hawthorne and his group in the period 1965-74 and have since been extensively exploited and extended by several groups. Examples of each will be given before mentioning some of the more recent routes that have been developed. It is worth noting that the carborane dianions (80) and (81) are both more effective as ligands than is perhaps because of 

When the reaction is carried out in the presence of NaCsHs the purple mixed sandwich complex 

The reaction is general and has been applied to many transition metals as well as lanthanides and actinides. Variants use metal carbonyls and other complexes to supply the capping unit, e.g. 

The reaction, which is quite general for closo-carboranes, involves the reductive opening of an n-vertex c/oio-cluster followed by metal 

The structure of the bimetallic 10-vertex cluster was shown by X-ray diffraction to be (84). When the icosahedral carborane l,2-C2BioHi2 was used, the reaction led to the first supraicosahedral metallocarboranes with 13- and 14-vertex polyhedral structures (85)-(89). Facile isomerism of the 13-vertex monometallodicarbaboranes was observed as indicated in the scheme above (in which = CH and O = BH). 

Polyhedral contraction is not so general a method of preparing metallocarboranes as is polyhedral expansion since some metallocarboranes degrade completely under these conditions. 

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Numerous other aluminacarborane structural types have also recently been synthesized by a variety of routes and, indeed, the burgeoning field of metallocarborane chemistry now encompasses the whole Periodic Table with an almost bewildering display of exotic and unprecedented structural types. 

Electron-counting Rules for Metallocarboranes and Other Heteroboranes... 

To the structural generalizations on carborancs (p. 183) can be added the rule that, in metallocarboranes, the M atom tends to adopt a vertex with high coordination number. M occupancy of a low CN vertex is not precluded, particularly in kineiically controlled syntheses, but isomerization to more stable configurations usually results in the migration of M to high CN vertices. 

In general, metallocarboranes are much less reactive (more stable) than the corresponding metallocenes and they tend to stabilize higher oxidation states of the later transition metals, e.g, [Cu (1,2-C2B9Hii)2]2- and [Cu (l,2-C2B9Hii)2] are known whereas cuprocene... 

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. 

Recent advances in the chemistry of small cage main group metallocarboranes 99JOM(581)13. 

Metallocarboranes structural studies in recent years. A. I. Yanovskii, Russ. Chem. Rev. (Engl Transl.), 1985, 54, 515 (78). 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

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 ... 

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, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

However, separation of the carborane cage from The phos-phazene ring or chain by a methylene spacer group allows metals to be Inserted into the open face of the carborane. These syntheses were accomplished by the reaction routes shown in Schemes 3 and A. High polymeric analogues of these transformations have also been accomplished following polymerization of XXX. The rhodium-bound cyclophosphazenes and polyphosphazenes are catalysts for the hydrogenation of 1-hexene. In this, they show a similar behavior to metallocarboranes linked to polystyrene... 

The crystal structure of the chromium metallocarborane salt Cs[Cr(BgC2-H9Me2)2] has been determined. The anion consists of two icosahedra sharing a chromium atom as a common vertex. ... 

Fig. 16. Some metallocarboranes and a metalloborane (metal ligands and exo-hydrogen atoms omitted). (THF, tetrahydrofuran.)...

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