Cluster compounds carbon-centered

The idea (50, 5/) of dual coordination of CO implies the presence of two coordination centers in a Fischer-Tropsch catalyst system, i.e., a carbonyl carbon coordinating center, Ma, and a carbonyl oxygen coordinating center, M6 (14). It is this concept which has led at least two groups to examine transition metal carbonyl cluster compounds as homogeneous Fischer-Tropsch catalysts. 

Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

Figure 1.3 shows the structures of some deltahedral (i.e., triangular-faced polyhedral) carboranesmixed hydride clusters of boron and carbon with BBB, BBC, or BCC faces. Each carbon atom in these cluster compounds has a hydrogen atom attached to it by a bond pointing away from the center of the cluster, but otherwise uses its three remaining valences to bond to the... 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

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. 

Ziebarth, R. P. Corbett, J. D. (1987). Cation Distribution within a Cluster Framework. Synthesis and Structure of the Carbon- and Boron-Centered Zirconium Cluster Compounds KZrgCl s and CsKZrgCl gB, J. Am. Chem. Soc. 109, 4844-4850. 

Similar to alkenes, the modes of bonding of alkynes in clusters is more complex than in monuclear compounds. In metal clusters, alkyne carbon atoms are normally bonded to two, three or four metal atoms. It is apparent that the interaction with the metal atoms produces an extensive orbital rearrangement on the carbon centers. Carbon-carbon distances are in this process substantially lengthened in some cases distances near to those of a single carbon-carbon bond are observed. The scheme in Fig. 2.8 shows basic structures for alkyne... 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

One of the simplest biochemical addition reactions is the hydration of carbon dioxide to form carbonic acid, which is released from the zinc-containing carbonic anhydrase (left, Fig. 13-1) as HC03-. Aconitase (center, Fig. 13-4) is shown here removing a water molecule from isocitrate, an intermediate compound in the citric acid cycle. The H20 that is removed will become bonded to an iron atom of the Fe4S4 cluster at the active site as indicated by the black H20. An enolate anion derived from acetyl-CoA adds to the carbonyl group of oxaloacetate to form citrate in the active site of citrate synthase (right, Fig. 13-9) to initiate the citric acid cycle. 

The C02-bridged polymetallic complexes involve coordination of the carboxyl carbon to one metal and bonding of one or two carboxyl oxygens to a second (or third) metal center, leading to compounds of the gm-T n type, as described in Figure 4.2. The first bridged C02 complexes to be structurally characterized were Fe-Re, Ir-Zr, and Rh-Os bimetallic complexes, or polymeric Co, Os, and Ru clusters (for a review, see Ref. [4] and references therein for details). 

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