Metal atoms carbon monoxide

An additional significance for carbidocarbonyl clusters has appeared more recently with the discovery of the fascinating reactivity of carbon atoms in clusters when they are exposed to reactive molecules in low nuclearity carbidocarbonyl clusters. These observations followed on the heels of the recognition of the crucial role played by surface bound carbon atoms in metal-catalyzed carbon monoxide hydrogenation, and so a new area of overlap between cluster chemistry and surface chemistry has arisen. Moreover, in this case the comparisons between organometallic and surface chemistry may lie in reactivity and not merely structural similarities. 

Dative covalent bonds, or coordinate covalent bonds, are those in which electrons are shared (as in all covalent bonds), but in which both electrons involved in each bond are contributed from the same atom. Such bonds occur in organometallic compounds of transition metals having vacant d orbitals. It is beyond the scope of this book to discuss such bonding in detail the reader needing additional information should refer to works on organometallic compounds.12 The most common organometallic compounds that have dative covalent bonds are carbonyl compounds, which are formed from a transition metal and carbon monoxide, where the metal is usually in the -1, 0, or +1 oxidation state. In these compounds the carbon atom on the carbon monoxide acts as an electron-pair donor ... 

When the metal-coordinated carbon monoxide in 12a is activated with a Lewis base like AlBr3, the colorless adduct 57 is isolated. This takes up carbon monoxide without elevated pressure at 20 °C. On hydrolysis and cleavage of the aluminum residue it is transformed into the ring-expanded, colorless, thermally labile insertion product 58. - Carbon monoxide is reeliminated starting at 20°C in solution. Scheme 20 reveals the first CO insertion into a Re—C a bond with an p -hybridized carbon atom. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

The initial step in the homogeneous-catalyzed water gas shift reaction undoubtedly involves nucleophilic attack of hydroxide ion at the carbon atom of a metal-bound carbon monoxide. Muetterties has observed that [Re(CO)e] exchanges oxygen atoms with atoms in water, probably... 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Some transition metal atoms combined with uncharged molecules as ligands (notahiv carbon monoxide. CO) have a formal oxidation state of 0. for example Ni + 4CO Ni"(CO)4. 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Oxo Synthesis. Ad of the synthesis gas reactions discussed to this point are heterogeneous catalytic reactions. The oxo process (qv) is an example of an industriady important class of reactions cataly2ed by homogeneous metal complexes. In the oxo reaction, carbon monoxide and hydrogen add to an olefin to produce an aldehyde with one more carbon atom than the original olefin, eg, for propjiene ... 

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). 

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. 

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

For many years the corrosion of uranium has been of major interest in atomic energy programmes. The environments of importance are mainly those which could come into contact with the metal at high temperatures during the malfunction of reactors, viz. water, carbon dioxide, carbon monoxide, air and steam. In all instances the corrosion is favoured by large free energy and heat terms for the formation of uranium oxides. The major use of the metal in reactors cooled by carbon dioxide has resulted in considerable emphasis on the behaviour in this gas and to a lesser extent in carbon monoxide and air. 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

FIGURE 14.35 Carbon monoxide can bind to a c/-metal atom in two ways ... 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

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