Carbon monoxide, reaction with metal

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

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). 

The Lewis definition of a base is broader than the Bronsted definition. That is, although every Bronsted base is a Lewis base, not every Lewis base is a Bronsted base. For instance, carbon monoxide is an important Lewis base in its reactions with metals, but it is not a Bronsted base because it does not accept protons. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

Certain apparently solid—solid reactions with a solid product are, in reality, solid—gas reactions. Thus, the reduction of a metal oxide by solid carbon is really a two stage process, the oxidation of carbon by gaseous carbon dioxide to form carbon monoxide, followed by the reduction of the metal oxide by the carbon monoxide to form metal plus carbon dioxide. Often, the carbon oxidation is the rate-controlling reacion and the rate of this reaction can be catalysed by the addition of small amounts of alkali and also by fine metal particles produced as a result of the reduction reaction [7]. 

Figure C shows carbon monoxide insertion reactions. There are a number of reduction reactions of carbon monoxide catalyzed by transition metals, and these, I believe, all involve an insertion of carbon monoxide into a metal hydride as an initial step. Cobalt hydrocarbonyl reacts with carbon monoxide to give formate derivatives. This is probably an insertion reaction also.

J. L. Gay Lussac and L. J. Thenard 5 showed in 1811 that if many of the metallic oxides be intimately mixed with carbon the reaction with chlorine proceeds more readily than with the oxide alone the metal chloride and carbon monoxide or dioxide are the products of the reaction. M. le Quesneville and F. Wohler used this process for aluminium chloride, chromic chloride, silicon tetrachloride, etc., and C. Baskerville for thorium tetrachloride. 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

Lanthanides as modifiers to other oxides in aluminas In zirconias In iron oxide Lanthanide oxides in mixed oxides With aluminas With iron oxides With other transition metal oxides To maintain surface area To increase oxidation rates To increase methanation rates For conduction in electrocatalysis For ammonia synthesis promotion To provide sulfur oxides (SO.,) control For dehydrogenation in carbon monoxide reactions For oxidation... 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

A synthetically useful technique for substituting (C-electrophilic) carbon monoxide ligands from metal carbonyls is a base reaction with the sufficiently nucleophilic trimethylamine oxide [8]. As represented by eq. (6), the same mechanistic scheme applies, with the zwitterionic intermediate breaking down into the (volatile, weakly coordinating) fragments CO2 and N(CH3)3 at the same time, a reactive fragment (e. g., the 16e-system Fe(CO)4 ) is being formed. 

As is common for many of the reactions with metal oxides and phosgene, the reaction rate goes through a maximum at ca. 650 C, drops to a minimum at ca. 850 C, and then starts to rise again (see Fig. 9.10). This is reasonably explained in terms of phosgene dissociation into carbon monoxide and dichlorine (see Chapter 8) becoming appreciable at above 600 C,... 

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