Cage Metallacarbaboranes

Discussion of these compounds in two sections. The first deals with reports emphasising synthesis and characterisation of novel compounds, the second concerns compounds of special interest in catalysis. 

Compounds in this section are discussed in order of increasing complexity of the carbaborane unit. 

The simplest metallacarbaborane reported was obtained from reaction of a BH fragment with two [ tnCp3W(iCMe3CCO 323 

The air-stable, paramagnetic, chromium compound was the first reported Itropylium)-metal(carbaborane) sandwich complex. 

Both CgB2 and CgB -based systems have been used to construct multidecKer sandwich compounds.Reaction of bls(allyl) nickel and 

Reactions of thermally generated Fe, Co, or Ni atoms with nldo-2,6-C2B7H11 in the presence of hydrocarbons (C5H6, tolH, 

The X-ray structural characterisation of [6,9-w- Pt(PPhs)2 -6,9-C2BgHio] has been reported.The Pt atom adopted a square planar configuration in its bridging position. 

The comaio[3,3 -Si (3,1,2-S1C2B9H21 ) complex (23), was prepared from SiCl4 and LI2 [7,8-C2B9H11J. It is isoelectronic with the comma section of (21). Reaction of (23) with two equivalents of Ll[Bu°] selectively deprotonated two C-H bonds. 

The structures (x-ray) of three partly halogenated commo-[3,3 -Co(3,1,2-C0C2B9H11)2] complexes have been reported. 

None exhibited the disorder present In the parent complex and In 

---

In addition to the areas already discussed in this article and, in particular, development of applications in biological, medicinal, and materials chemistry, several aspects are of current research interest. Section 4.10 introduced the steadily expanding area of single-cage metallacarbaborane clusters with more than 12 vertices. The role of carbaboranes and metallacarbaboranes within supramolecular chemistry has been actively investigated in the last decade. One of the best-explored areas is that of multicluster mercuracarbaboranes, which act as macrocyclic Lewis acidic hosts for electron-rich (anionic or neutral) guests. ... 

Figure 1 Closed deltahedra (polyhedra with triangular faces) with 5-12 vertices and the lUPAC numbering system for each cage. The structures of most metallacarbaboranes are based on or derived from these eight deltahedra...

In most metallacarbaboranes, the interaction between the metal atom and carbaborane cage is considered to be essentially covalent, and this is supported by the experimentally determined M-B and M-C distances. However, in [3-Tl-l,2-C2B9Hii] , the distances from the thallium(I) center to the C and B atoms in the open face of the C2B9 cage are particularly... 

In some metallacarbaborane clusters, a metal fragment is supported on the outside of the carbaborane skeleton, typically by B H M interactions, for example, complex (27) involves exo interactions between the T1(I) atom of one cage and two terminal boron hydrogen bonds of a second cage. The exo descriptor illustrates the relationship of the metal fragment to the carbaborane and the nido descriptor classifies the carbaborane cage. 

Exo-nido metallacarbaborane complexes are at present comparatively rare, but are nevertheless a clear bonding option by which a number of metal-ligand fragments may interact with a C2B9 cage system. However, the species 62 are singled out from all other known examples by having... 

Fig. 9. Typical 13C- H NMR spectrum, showing the cage-carbon resonances, of a 12-vertex metallacarbaborane complex having a 2,1,8-MC2B9 arrangement of atoms.

For C-derivatized metallacarbaborane clusters, the substituents attached to the cage carbon atoms are usually introduced into the carbaborane precursor. Examples have been given earlier in equations (4), (5), (8), and (12), and Scheme 4. 

---