Carbon monoxide, bond dissociation

Carbon monoxide eventually dissociates at room temperature on all but some of the group 8-10 (VII IB) metals (44). This dissociation occurs only for metal surfaces which form sufficiently strong metal-carbon plus metal-oxygen bonds to break the 257 kcal/mole CO bond. The known values for gas phase metal atoms predict the same trend(48). The similarity in the behavior of surfaces and atoms... 

The revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

The resulting reduced photoproduct acts as the catalytic intermediate for COj reduction (Figure 33). It has been suggested that carbon monoxide ligand dissociation, followed by formation of a rhenium-formate intermediate, leads to cyclic reduction of carbon dioxide to CO. Interestingly, a rhenium(I)-carboxylate complex, /ac-Re(0—C(=0)—H)(bpy)(CO)3, has been isolated as a by-product of the photosystem. The latter complex is inactive in COj reduction to CO, and hence a rhenium-formate intermediate formed by COj insertion into the hydride bond was suggested as the active intermediate for CO formation (Figure 33). 

In structure II (numbered 13 in the IRC output), the C-H bond has lengthened with respect to the transition structure (1.23 versus 1.09A), while theC-O bond length has contracted slightly. Both changes are what would be expected as formaldehyde dissociates to form carbon monoxide and hydrogen molecule. ... 

Step through the sequence of structures representing dissociation oiketene to methylene and carbon monoxide. Plot energy (vertical axis) vs. carbon-carbon bond distance (horizontal axis). Would you describe ketene as a weak complex between singlet methylene and carbon monoxide Explain. (A table of CC and CO bond lengths is found at left.) Is there an energy barrier to the dissociation ... 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

The mode of chemisorption of CO is a key-factor concerning selectivity to various products. Hydrocarbons can only be produced if the carbon-oxygen bond is broken, whereas this bond must stay intact for the formation of oxygenates. It is obvious that catalysts favoring the production of hydrocarbons must chemisorb carbon monoxide dissociatively (e.g. Fe) while those favoring the formation of oxygenates must be able to chemisorb carbon monoxide molecularly (e.g. Rh). 

At the outset it is important to clarify the scope of this discussion by the elimination of areas which will not be considered. When one notes that the term "activation of carbon monoxide" may mean a process as little perturbative of the C-0 bond as its end-on attachment to a metal atom in carbonyls, or as strongly perturbative as its dissociation to atoms on a metal surface, the need for limits becomes obvious. In this discussion we will consider only the activation of carbon monoxide in the sense that isolable products are formed by the addition of hydrogen to the molecule without complete rupture of all carbon-oxygen bonds, oxygenates are formed. 

Carbon Monoxide. There are close similarities between carbon monoxide and nitrogen. The molecules are isoelectronic, and the bond lengths and dissociation energies are quite comparable. The phase diagrams of the two compounds show the same trends in the moderate pressure range with a variety of phase transitions between essentially alike crystal structures [333], when allowance is made for the lack of the inversion center and the presence of a weak electric dipole moment in carbon monoxide. However, the behavior and stability at higher... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

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