Metal carbonyls oxidation

L oss of Catalyst by Vapor Transport. The direct volatilisation of catalytic metals is generally not a factor in catalytic processes, but catalytic metal can be lost through formation of metal carbonyl oxides, sulfides, and hahdes in environments containing CO, NO, O2 and H2S, and halogens (24). 

Lewis acid treatment of 1,2,4-trioxolanes gives metallated carbonyl oxides which may be trapped by cycloaddition to allylsilanes to give 1,2-dioxolanes 78 <99TL6553>. 

Photochemistry of transition metal carbonyl complexes as the borderline between organic and inorganic chemistry is mentioned in Section 6.3.9. Since the dissociation energy of a common metal carbonyl oxide bond is usually low, photodecarbonylation, that is, release of the CO molecule, is the most common photoprocess observed (entry 8). 

Both UV and visible light have sufficient energy to initiate many processes in coordination complexes. Decarbonylation is one of the most typical photoreactions, because the dissociation energy of a common metal carbonyl oxide bond is as low as 200kJ mol 1.1048 Scheme 6.154 presents two examples the fission of (a) metal—CO1049 and (b) metal—CO—alkyl1050 bonds in some carbonyl complexes. In the latter case, irradiation of an enantiomerically pure iron complex 348 leads to decarbonylation, which is followed by alkyl migration. 

In contrast to the metal carbonyl oxidations described above is the reduction of Cp2Fe2(CO)4 with [Cp 2ZrN2]2N2 (263). The product of this reaction is a species in which a C—C bond is formed between two carbonyl groups [Eq. (59)]. The C—C distance, 1.57 A, is consistent with formation... 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Because transition metals even in a finely-divided state do not readily combine with CO, various metal salts have been used to synthesize metal carbonyls. Metal salts almost always contain the metal in a higher oxidation state than the resulting carbonyl complex. Therefore, most metal carbonyls result from the reduction of the metal in the starting material. Such a process has been referred to as reductive carbonylation. Although detailed mechanistic studies ate lacking, the process probably proceeds through stepwise reduction of the metal with simultaneous coordination of CO (90). 

The dry method for synthesizing metal carbonyls from salts and oxides has proven very usehil in a number of cases. The metal carbonyl is formed in the presence of a suitable reducing agent. In some cases CO itself is the reducing agent. Rhenium (97) and technetium (98,99) carbonyls are conveniently... 

Most metal carbonyls are synthesized in nonaqueous media. Reactive metals, such as sodium (85), magnesium (105), zinc (106), and aluminum (107,108), are usually used as reducing agents. Solvents that stabilize low oxidation states of metals and act as electron-transfer agents are commonly employed. These include diethyl ether, tetrahydrofiiran, and 2-methoxyethyl ether (diglyme). 

The main synthetic route to high nuclearity metal carbonyl clusters involves a condensation process (/) a reaction induced by coordinatively unsaturated species or (2) a reaction between coordinatively saturated species in different oxidation states. As an example of (/), Os2(CO)22 can be condensed to form a series of higher coordinated species (89). 

The nature of the bonding, particularly in CO, has excited much attention because of the unusual coordination number (1) and oxidation state (-f2) of carbon it is discussed on p. 926 in connection with the formation of metal-carbonyl complexes. 

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

Metal carbonyls form several complexes but those of major interest in MOCVD are the carbonyl halides and the carbonyl-nitric-oxide complexes. 

Carbonyl Nitric Oxides. Another group of metal-carbonyl complexes, worthy of investigation as CVD precursors, consists of the carbonyl nitric oxides. In these complexes, one (or more) CO group is replaced by NO. An example is cobalt nitrosyl tricarbonyl, CoNO(CO)3, which is a preferred precursor for the CVD of cobalt. It is a liquid with a boiling point of 78.6°C which decomposes at 66°C. It is prepared by passing NO through an aqueous solution of cobalt nitrate and potassium cyanide and potassium hydroxide. ... 

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. 

Keywords Oxide surfaces Metal carbonyls Ziegler-Natta catalyst IR spectroscopy EPR spectroscopy... 

In the following review we will focus on two classes of systems dispersed metal particles on oxide supports as used for a large variety of catalytic reactions and a model Ziegler-Natta catalyst for low pressure olefin polymerization. The discussion of the first system will focus on the characterization of the environment of deposited metal atoms. To this end, we will discuss the prospects of metal carbonyls, which may be formed during the reaction of metal deposits with a CO gas phase, as probes for mapping the environment of deposited metal atoms [15-19]. 

We have reviewed experiments on two classes of systems, namely small metal particles and atoms on oxide surfaces, and Ziegler-Natta model catalysts. We have shown that metal carbonyls prepared in situ by reaction of deposited metal atoms with CO from the gas phase are suitable probes for the environment of the adsorbed metal atoms and thus for the properties of the nucleation site. In addition, examples of the distinct chemical and physical properties of low coordinated metal atoms as compared to regular metal adsorption sites were demonstrated. For the Ziegler-Natta model catalysts it was demonstrated how combination of different surface science methods can help to gain insight into a variety of microscopic properties of surface sites involved in the polymerization reaction. 

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