Carbide clusters bonding

We have shown that A) interstitial hydride formation is observed only with partial occupation of the available holes, B) occupation of the interstitial position in isolated polyhedra is not observed, and C) occupation of all the holes in a close-packed lattice cancels metal-metal interactions. Therefore, it seems that interstitial hydrogen can be tolerated only in a fraction of the total number of holes, and with the weakening of metal-metal interactions. This behavior indicates strong competition between metal-metal and metal-hydrogen bonds, which is unique for hydrogen because interstitial carbon can stabilize some unusual arrangements in carbonyl carbide clusters (29, 30). 

Despite the existence of 86-electron octahedral carbide clusters, such as Ru6(CO)i7C, the dianion [Rhf,(CO) uG]2 has 90 valence electrons and a trigonal prismatic array of metal atoms (structure XVIII in Fig. 8) (7). This fact could indicate that the three M—M bonds formally lost in the... 

Fig. 7. A chiral rhodium carbide cluster (49) and its K5 subgraph (50). The atoms chosen as the vertices of 50 have been numbered in 49 (the Rh —C bonds have been neglected for clarity)...

Cluster modeling of possible chemisorption states and of possible intermediate states in surface reactions can to a first approximation be useful in guiding experiments or interpretations of experimental data for surface reactions (23-25). One important and enlightening result (6, 26, 27) in metal carbide cluster chemistry will be used here to illustrate this particular point because it bears directly on the importance of multicenter C-H-M bonding for hydrocarbon fragments in metal chemistry. 

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. 

Because the enthalpies of formation of such molecular metal carbonyl carbide clusters have yet to be measured or calculated accurately, it has not been possible to calculate their metal-carbon bond enthalpies. However, enough thermochemical information is available on some bulk metal carbides to allow the strengths of both their metal-metal and metal-carbon bonds to be assessed, as indicated in the next section. 

The interconversion between octahedral and trigonal prismatic is of relatively high energy because an octahedron has 12 metal-metal bonds and a trigonal prism has only nine metal-metal bonds and, therefore, three bonds must be broken or formed. Such processes are quite rare and examples of reversible processes are limited to a few carbide clusters of the Group 9 metals. 

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

Shibuta, et al. Bond-Order Potential for Transition Metal Carbide Cluster for the Growth Simulation of a Single-Walled Carbon Nanotube. Department of Materials Engineering, The University of Tokyo 2003. 

Driess, Matthias, Silicon-Phosphorus and Silicon-Arsenic Multiple Bonds Dyson, Paul J., Chemistry of Ruthenium-Carbide Clusters RusC(CO)i5... 

In contrast to [Fe4H2(CO)i3], the analogous hydrides of ruthenium or osmium are stable. Besides tri- and tetranuclear clusters, also known are clusters containing five or six iron or ruthenium atoms and from five to eight osmium atoms as well as carbonyl carbide clusters in which metal atoms are bonded to the carbon atom (Figure 2.22) [Fe5(CO)i5C], [Fcs CO),CV-, [Ru5(CO)i5C],... 

IR spectra of polynuclear and cluster carbonyls were discussed in Chapter 2. The v(M—C) frequencies in carbide clusters occur in the 500-850 cmrange, and therefore are usually higher than the v(M —CO) frequencies in carbonyls. The A (Co —C) force constant for the Co —C bond in [Co6C(CO)i2S2] is 155 N and is lower than... 

Metal carbonyl carbide cluster compounds may be formed either by the carbon-oxygen bond breaking of the CO molecule or by the decomposition of an organic compound present in the reaction mixture. Chloroform is a convenient source of the encapsulated carbon atom because in this case the synthesis may be carried out under mild conditions ... 

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