Metal carbon monoxide bonding

Fig. 1. (a) Formation of a ( -bond between a transition metal and carbon monoxide, (b) Metal-to-carbon monoxide TT-bond formation. 

Second, the idea of the ejection of a saturated fragment is compatible with the established view of the bond enthalpies for metal-carbon monoxide > metal-metal. Thus the formation of M-CO bonds at the expense of M-M bonds is to be anticipated. 

Density functional models provide a much better account. The local density model does the poorest and BP and B3LYP models do the best, but the differences are not great. As with metal-carbon (carbon monoxide) lengths, bond distances from all-electron 6-3IG calculations are usually (but not always) shorter than those obtained... 

Metal-carbon monoxide ligand bond lengths in these same compounds have already been provided in Table 5-11. 

In carbon monoxide the bond between the atoms depends, as in the N2O molecule, on an asymmetrical electron shift, electrons of the 0 atom moving toward the C atom, and the CO molecule having a dipole character. In this case, too, metal electrons are displaced toward the adsorbed molecule and taken from the electron gas, as shown by the change of the electrical resistance of thin nickel films on carbon monoxide adsorption (18). 

Insertion reactions with carbon monoxide or sulfur dioxide are well known for compounds with carbon-transition-metal bonds, e.g.,... 

Most carbonyl compounds have several carbon monoxide molecules bonded to a metal. 

The surface coverage of an adsorbate is another important parameter in ordering. We shall use the common definition of coverage where one monolayer corresponds to one adsorbate atom or molecule for each unit cell of the clean, unreconstructed substrate surface. Thus, if an adsorbed undissociated carbon monoxide molecule bonds to alternating top-layer metal atoms exposed at the Ni(100) surface, we have a coverage of a half monolayer. 

The mechanism of hydrosilylation on a transition metal catalyst involves an intermediate with a carbon-transition metal bond. Carbon monoxide, if present, inserts into the carbon-metal bond to give an... 

Carbon monoxide-metal systems have also been used to demonstrate the effect of carrier material on properties of supported metal catalysts. Eischens and Plisken (1) found that the ratio of concentrations of linear to bridged species was much larger when platinum was supported on silica than when it was supported on alumina. Carbon monoxide on the alumina-supported samples was much more difficult to oxidize than on silica-supported ones. These observations were interpreted as an enhancement of platinum s ability to donate electrons to the metal-CO bond when alumina is the support. However, the mechanism by which... 

Metal carbonyls belong to a unique class of organometallic compounds where carbon monoxide is bonded to the metal atom through the carbon end. They enjoy their relevance in the synthesis of various complex compounds and cluster compoxmds as well as a potential agent in organic transformations and occasionally catalyze some of the unique chemical transformations. 

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

A weakened carbon—oxygen bond results from the combined ( - and TT-bonding, allowing the metal-bonded carbon monoxide to react more readily... 

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

Interaction of the iron metal atoms with thiophenes (thiophene, 2-methyl-, and 2,5-dimethylthiophene) in the vapor phase at 77 K with subsequent heating in a carbon monoxide atmosphere also leads to the formation of ferrole 83 [76JOM(l 18)37, 77CJC3509]. The iron cyclopentadienyl ring is planar and all the bonds have multiple character. 

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