Carbon interaction with metal atoms

The interaction of metal atoms with carbon-halogen bonds is characterized by oxidative cleavage of that linkage and is discussed more fully in Section IV. An exception to this generalization appears to be perfluorobut-2-ene which reacts with transition metal atoms in a manner similar to C2H4 and norbornene (54, 55) ... 

Prior to a discussion of C02 insertion reactions into M-H and M-C bonds it is useful to review some of the known coordination chemistry of carbon dioxide, since activation of COz by metal centers is assumed to be of significance in most of these processes. Carbon dioxide can interact with metal centers by three functionalities. These include the Lewis acid site at carbon (1), the Lewis base sites of the terminal oxygen atoms (2), and the t]2 C=0 bond (3). It is possible as well that a combination... 

Among the enzymes that occur in biological systems and which utilize C02 as a source of carbon, some contain metal atoms as the active center, where C02 is converted (as in RuBisCO or biotin-dependent carboxylases). However, some of these enzymes do not require metal, or, if so, it does not interact directly with C02. 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

The bonding in monometal alkyne complexes is usually interpreted in terms of the Dewar-Chatt-Duncanson model (293), since the alkyne molecule has a pair of n and n molecular orbitals which lie in the plane of the metal and the two carbon atoms. These two orbitals are denoted n and n, and are analogous to those in jr-bonded alkene complexes (394). There is also a pair of n and n molecular orbitals which lie perpendicular to the metal-carbon plane, denoted nL and n . These orbitals are illustrated in Fig. 14. Both sets of n and n orbitals have the correct symmetry to interact with metal d orbitals. The interaction... 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

A key question is whether the diatomic molecule in its interaction with metal surfaces remains molecular or dissociates into carbon and oxygen. Broden et al. (3) predicted, by the perturbation of molecular orbitals for CO adsorbed, that only iron could dissociate CO. However, other metals in Group VIII such as nickel (A) ruthenium (5) and rhodium (6) can dissociate CO. Recently Ichikawa et al.(7) observed that disproportionation of CO to CO2 and carbon occurs on small particles of silica-supported palladium. These results show that carbon deposition phenomena may occur via either dissociation of CO on the metals used or disproportionation of CO to CO and carbon on small platinum particles. Cant and Angove (8) studied the apparent deactivation of Pt/Si02 catalyst for the oxidation of carbon monoxide and they suggested that adsorbed CO forms patches and that oxygen atoms are gradually consumed. 

Although K electron interactions seem to be important in desulfurization, both sulfur-containing and non-sulflir aromatic compounds interact with metals on the catalyst support (MCM alumina, activated carbon, zeolites) via this mechanism. Specific for thiophenic compounds are only two types of interactions, which can be used for separation. They are the t) -S bonding interactions between the sulfur atom and one metal atom, and the S-ps bonding interactions between the sulfur atom and two metal atoms [7], They are illustrated in Fig. 36... 

The anomalous stretching frequency has been attributed to interaction of the endo-C—Hoc with the metal its absence in the exo-substituted phenylcyclopentadiene cobalt complex (Figure 19) has been explained by an increased bending-away of the methylene carbon from the metal atom which makes the enrfo-hydrogen-metal distance too large for appreciable metal-hydrogen interaction [56]. 

It is worth to notice that those complexes are the first group 13 methanediide complex featuring a dianionic carbon interacting with one sole metal atom and the first indium and gallium methanediide complexes. 

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