Carbon-centered clusters

Encapsulated atoms contribute their valence electrons to the total electron count. For example, carbon contributes its 4 valence electrons in RugC(CO)i7 to give a total of 86 electrons, corresponding to a closo electron count (Table 15.12). 

EXERCISE 15.9 Classify the following clusters by structural type  

FIGURE 15.23 Carbide Clusters. CO ligands have been omitted for clarity. 

FIGURE 15.24 Bonding Interactions between Central Carbon and Octahedral RUg. 

FIGURE 15.25 Examples of Large Clusters. CO and hydride ligands have been omitted to show the metal-metal bonding more clearly. 

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Carbon itself may play quite a different role than commonly encountered in organic chemistry. Certain metal clusters encapsulate carbon atoms the resulting carbon-centered clusters, frequently called carbide clusters, in some cases contain carbon bonded to five, six, or more surrounding metals. The traditional notion of carbon forming bonds to, at most, four additional atoms, must be reconsidered. Two examples of carbide clusters are included in Figure 13-2. 

Using molecular modeling software, draw the core carbon-centered cluster of Ru5C(CO)]7 (see Figure 15-22), and calculate its molecular orbitals. Identify and display the following ... 

Carbon plays an unusual role in a number of metal cluster compounds in which a carbon atom is at the center of a polyhedron of metal atoms. Examples of carbon-centered clusters with five, six, or more surrounding metals are known (Figure 1.7). The striking role that carbon plays in these clusters has provided a challenge to theoretical inorganic chemists. 

Many compounds have been synthesized, often fortuitously, in which one or more atoms have been partially or completely encapsulated within metal clusters. The most common have been the carbon-centered clusters, also called carbide or carbido clusters, with carbon exhibiting coordination numbers and geometries not found in classic organic molecules. Examples of these unusual coordination geometries are shown in Figure 15.23. 

Draw the core carbon-centered cluster of RugCfCOfn... 

FIGURE 22.5 Schematic view of kinetically aggregated filler clusters in mbber below and above the gel point <1>. The left side characterizes the local stmcture of carbon black clusters, built by primary particles and primary aggregates. (Every black disk in the center figure [ and on the right-hand side

primary aggregate.) (From Kliippel, M. and Heinrich, G., Kautschuk, Gummi, Kunststojfe, 58, 217, 2005. With permission.)... 

Rosch, N., Gorling, A., Ellis, D.E. and Schmidbaur, H. (1989) Aurophilicity as Concerted Effect Relativistic MO Calculations on Carbon-Centered Gold Clusters. Angewandte Chemie International Edition, 28, 1357-1359. 

A similar reaction pathway was found for the Sn2 substitution of an epoxide with a lithium cuprate cluster [124]. In contrast to that in the MeBr reaction, the stereochemistry of the electrophilic carbon center is already inverted in the transition state, providing the reason for the preferred trans-diaxial epoxide-opening widely observed in synthetic studies. The TS for the Sn2 reaction of cyclohexene oxide is shown in Eq. 10.12. 

The following gas-phase reactions of anions have been studied and will be briefly reviewed in the next sections proton transfer reactions, nucleophilic displacement reactions at both aliphatic and aromatic carbon centers, elimination reactions, electron transfer (ET) reactions, reactions with carbon-carbon double bonds and carbonyl functions, and association or complex (cluster)-forming reactions of various types. 

In contrast to hydroformylation of olefin derivatives, the addition of carbon monoxide and trialkylsilane to alkynes gives carbon-centered silanes exclusively when catalyzed by, for example, rhodium and Rh-Co carbonyl clusters [153, 154] and Rh2(pfb)4 (pfb = perfluorobutyrate) (eq. (13) [153]). 

Carbon-and Nitrogen-Centered Clusters (Adapted with permission from G.L. Miessler and D.A.Tarr, Inorganic Chemistry, 3rd ed., Pearson Education Upper Saddle River, NJ, 2004, p. 587.)... 

The main oxidation reactions of the 2-deoxyribose of DNA are mediated by "OH that are able to abstract hydrogen atoms from most of the positions with the exception of the methylene group at C2, which is a poorly reactive site [12, 14, 112], An abundant literature is available on the degradation pathways that are derived from the reactions of osidic carbon-centered radicals and that lead in most cases to the formation of strand breaks [12, 14, 112], However, there is one major exception that concerns the chemical reactions of the CT radical that is the precursor of 2-deoxyribonolactone [113]. Here, emphasis is placed on reactions of C4 and C5 radicals that may lead to the formation of base modifications either as tandem lesions or clustered damage. 

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