Closo isomers

It has beennoted by workers in both the USSR (178-181) and United States (24, 31, 42) that all closo isomers of CgBioHjg (II-C2, III-C2, and... 

Fairly close polydispersity values of polymer samples prepared in the presence of complexes 1 and 2 having different structures may be explained by the transformation of exo-nido complex 1 into its closo isomer 2, which is known to proceed in high yield at 80 °C 14). This transformation does not occur for the more sterically hindered C,C-dimethyl-substituted exo-nido complex 7, and, in this case, a polymer with a higher polydispersity index (M /Mn = 1.93) is formed. This finding indicates that the propagation step and the molecular-weight characteristics of PMMA are affected by both steric and stractural features of the rathenacarborane catalysts. 

For example, pyrolysis of the previously mentioned nido-2,3-C2B4Hg gives the 3 c/o o-species shown above, whereas under the milder conditions of photolytic closure the less-stable isomer closo-, 2-C2 i is obtained. Pyrolysis of alkyl boranes at 500-600° is a related route which is particularly useful to monocarbaboranes though the yields are often low, e.g. ... 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

Scheme 1 also shows that this ortho-cycloboronated iso-closo iridium(v) compound, can be formed directly and quantitatively by the mild thermolysis of compound which is the acyclic analogue of the cycloboronated isomers and Compound... 

Fig. 2.1-32. The two possible isomers of protonated monosubstituted closo-hexaborate.

Noth and Thorn reported the preparation of P4(NH)5[M(CO)5l4 with M = Cr and Mo, by thermal decomposition of P(NH2)3M(C0)5, and of P4(NH)5S4, through the reaction of Sg on the chromium adduct the mixed P4(NH)g[Cr(CO)5jjjS4 jj were identified during the process (62). Since the parent compound P4(NH)5 itself is still unknown, this exemplifies the possibility of obtaining new closo-structures, or less stable isomers of known ones, in a complexed form. 

Kesanli B, Fettinger J, Eichhom B (2001) The closo-[Sn9M(CO)3] - Zintl ion clusters where M = Cr, Mo, W two stmctural isomers and their dynamic behavior. Chem A Euro J 7 5277-5285... 

Fig. 14. The structure of one isomer of closo-(CO)5Co2B10Hg[S(C2H5)2]2 without the hydrogens.

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. 

In essence the alkyne has inserted across a metal-metal bond to give a closo-FeRu3C2 cluster. Such a structure is fully consistent with Wade s skeletal electron counting rules for a closo structure with 6 vertices (23,24). The three isomers of FeRu3(CO)i2(lfeCECPh) arise because in the equatorial isomer the methyl substituent can occupy a position cis or trans to the Fe atom. 

---