Twelve vertex

If one vertex and its attendant bonds are removed from a ball-and-stick model of the closo twelve-vertex icosahedron and two bonds are subsequently inserted into the open face, the eleven-vertex deltahedron results. If from each resulting smaller deltahedron any one of the lowest-coordination vertices, and its attendant bonds, are monotonically removed and one bond is inserted, the next smaller deltahedron results in all cases, from the icosahedron to the trigonal bip3U amid. It was the exact reverse of this primitive ball-and-stick degradation concept (process L) which allowed the correct bisdisphenoid 154) structure for C2BeHg (VI-02) to be anticipated 172) prior to its production. 

An attempt to relate the known closo-icosahedral parents to their nido twelve-vertex progeny follows. 

There is a special and very important feature of the anticipated open nido twelve-vertex structures in Fig. 12 repetition of single Lipscomb dsd rearrangements (denoted by the two-headed arrows) monotonically allows the six skeletal atoms about the open face to rotate about the second tier of five skeletal atoms (two-tier dsd rotation). Each dsd rearrangement [85, 163) (valence bond tautomerism) recreates the same configuration and involves only the motion of two skeletal atoms (in the ball-and-stick representation) and would allow carbons, if located in different tiers, to migrate apart. Such wholesale valence bond tautomerism is known to accompany the presence of seven-coordinate BH groups, e.g., and CBjoHu 142,155). 

XII- N12). Actually, carbon would prefer to be about the open face, which suggests that the ideal carbon positions upon the twelve-vertex nido skeleton would be those displayed in XIII-N12 and, given the fluxional characteristics that accompany seven-coordinate BH groups, intermediate XII-N12 should be able to rearrange into the more stable... 

Many other transition atoms simply substitute for the standard six-coordinate BH groups. Representative examples are in the seven-, eight-, nine- 45), ten-, eleven-, and twelve-vertex 50) close species displayed as V-, VI-, VII- VIII-, and 1-24 and the group IX-, X-, XI-, and XII-24, which illustrate the diversity possible. 

Polyhedral carboranes (continued) seven-vertex carboranes, 3, 58 six-vertex carboranes, 3, 56 small carborane syntheses, 3, 54 subicosahedral carborane geometrical patterns, 3, 50 subicosahedral and icosahedral, 3, 51 supraicosahedral, characteristics, 3, 96 ten-vertex carboranes, 3, 60 thirteen-vertex carboranes, 3, 100 tricarbaborane synthesis, 3, 54 twelve-vertex carboranes, 3, 98 Polyhedral skeletal electron pair theory, Ru and Os tetranuclear clusters, 6, 874-875 Poly(iV-heterocyclic carbene) ligands... 

Our approach to the subject has been to divide the metallocarboranes according to the size of the polyhedron. Starting with twelve-vertex compounds, which constitute the majority of the effort, we proceed to the larger polyhedra, so far unknown in the B H 2 and C2B 2H series, and then to the lower polyhedra. Further subdivisions within each polyhedral size include synthesis, structures, and properties of monometallic complexes, reactions of monometal lies, bimetallic preparations and reactions, and, in two instances, trimetallic compounds. 

It should be noted that in the polyhedral expansion process, as idealized in Eqs. (5) and (6), the product mctallocarborane has one more vertex than was present in the carborane starting material—hence the origin of the descriptive phrase polyhedral expansion. By contrast, wThen metallocarboranes are prepared by reaction with O2B9H112- ions, which are prepared from the icosahedral C2B10H12 carboranes, twelve-vertex metallocarboranes result. 

Degradation of the icosahedral 1,2- and 1,7-C2Bi0Hi2 isomers with strong base to produce the nido eleven-vertex anions 7,8- and 7,9-CoB9Hi2-has been previously discussed and is an important route to the preparation of twelve-vertex monometallocarboranes. The discovery that similar reactions could be performed on metallocarboranes led to the isolation of novel chains of metal atoms bridged by carborane groups and to the development of the polyhedral contraction and polyhedral subrogation reactions. 

Several other twelve-vertex metallocarborane carbonyl complexes have been prepared (51). In general, the chemistry and structures of these species, when investigated, have been found to parallel the analogous cyclopentadienyl metal carbonyls. 

Examples of bimetallic twelve-vertex metallocarboranes have been provided by various synthetic efforts. The preparation of bi- and trimetallic chain complexes by polyhedral subrogation of (1,2-C2B9Hn)2Co has been mentioned earlier (38, 34), and the Co(C2B8Hi0)Co fragment present therein may be considered as an example of this type of bimetallic complex. 

A second route to bimetallic twelve-vertex complexes is via polyhedral expansion. The starting material may be either an eleven-vertex mono-mctallocarborane,... 

The discovery of the thermal metal transfer reaction (27) afforded a third preparative route to bimetallic twelve-vertex metallocarboranes. This method involves the pyrolysis of eleven-vertex cfoso-metallocarboranes or cobalticinium salts of eleven-vertex cowimo-metallocarboranes and results in the production of several isomeric, closed, neutral, twelve-vertex bimetallocarboranes [Eqs. (9) and (10) Fig. 7]. Yields are reasonable in... 

Only one trimetallic twelve-vertex metallocarborane has been reported. This species, (C5H5)3Co3C2B7H9, arose as a side product during the polyhedral expansion of 2-C5H6-2-Co-l,6-C2B7H9 with Co(II) and C6H5 (25, 28). The isolation of this trimetallic complex suggests that the polyhedral expansion reaction may be extended to bimetallic substrates to produce novel metal-rich polyhedra. 

Thirteen-vertex complexes of metals other than cobalt have been prepared, as have commo thirteen-vertex anions and metal carbonyl derivatives (18). In general, these complexes exhibit stabilities similar to or less than their twelve-vertex counterparts. 

The chemistry of the thirteen-vertex monometallocarboranes, although less extensively studied, is similar to that of the twelve-vertex analogs. Thus, C5H6CoC2Bi Hi2 undergoes polyhedral subrogation to produce thirteen-vertex bimetallocarboranes [the metals may be identical (23) or... 

When subjected to the action of alcoholic base and excess Co(II) in the absence of additional cyclopentadienidc ion, the thirteen-vertex monometallocarborane reacts to produce a trimetallic complex that contains two bridging thirteen-vertex frameworks (23). The proposed structure of this species is shown in Fig. 17. This reaction is similar to that used to prepare the twelve-vertex bi- and trimetallic chain complexes discussed in Section III, B. 

Reactions of monometallic eleven-vertex metallocarboranes have been discussed in previous sections and may be summarized briefly as (a) polyhedral expansion to bimetallic twelve-vertex complexes and (6) thermal metal transfer to bimetallic twelve-vertex compounds. Polyhedral contraction to ten-vertex monometallocarboranes is discussed in Section VII. 

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