Carbon monoxide with organometallic

Transition-metal organometallic catalysts in solution are more effective for hydrogenation than are metals such as platinum. They are used for reactions of carbon monoxide with olefins (hydroformyla-tion) and for some ohgomerizations. They are sometimes immobihzed on polymer supports with phosphine groups. 

In both homogeneous and heterogeneous catalysis, carbon monoxide activation involves first the coordinaiive interaction of carbon monoxide with a metal acceptor center. Carbon monoxide, being a weak donor base, does not react with a proton and produces only a vety weak interaction with a hard acid center such as BH3, With less hard Lewis centers, such as CuX, AgX, AuXj etc. (X - halogen), more or less stable carbon monoxide adducts can be isolated. A variety of modes of CO coordination in well characterized organometallic complexes is known. Scheme 1 contains some selected examples. 

The use of supercritical fluids as reaction media for organometallic species is also investigated. Reactions include photochemical replacement of carbon monoxide with N2 and H2 in metal carbonyls, where the reaction medium is supercritical xenon. Also, photochemical activation of C-H bonds by organometallic complexes in supercritical carbon dioxide is investigated. More recent studies on photochemical reactions also include laser flash photolysis of metal carbonyls in supercritical carbon dioxide and ethane and laser flash photolysis of hydrogen abstraction reaction of triplet benzophenone in supercritical ethane and... 

Since the 1970s, palladium and platinum complexes with dibenzylideneacetone Pd(dba)2 and M2(dba)3 (M = Pd, Pt) have been known to react under mild conditions with either hydrogen or carbon monoxide, with the formation of a metal [211]. Indeed, there exists a long series of examples where CO and H2 have been used to decompose organometallic precursor molecules [173-177,179-181,183,184,186-188, 212-216]. As an example, the decomposition Ru(COD)(COT) (COD = cyclo-octadiene COT =cydo-octatriene) in an atmosphere of hydrogen is worthy of mention [189,190]. (Scheme 3.20). In this case, the precursor molecule is dissolved in a methanol-THF mixture and is contacted with H2 (3 bar pressure) at room temperature for at least 45 min. Depending on the nature of the MeOH-THF mixture, the Ru particle size can be designed between 3 and 86 nm. 

The 18-electron rule is obeyed so frequently with metal carbonyls that it can often be used to predict the products of the reaction of a metal with CO (g). For example, when Ni (which is d °) reacts with eous carbon monoxide, the organometallic product must contain four CO li ds in order to satisfy the... 

Batistini, A. Consiglio, G Mechanistic aspects of the alternating copolymerization of carbon monoxide with olefins catalyzed by cationic palladium complexes. Organometallics 1992,11,1766-1769. 

Low oxidation states - An important characteristic of transition metal chemistry is the formation of compounds with low (often zero or negative) oxidation states. This has little parallel outside the transition elements. Such complexes are frequently associated with ligands like carbon monoxide or alkenes. Compounds analogous to Fe(CO)s, [Ni(cod)2] (cod = 1,4-cyclooctadiene) or [Pt(PPh3]3] are very rarely encountered outside the transition-metal block. The study of the low oxidation compounds is included within organometallic chemistry. We comment about the nature of the bonding in such compounds in Chapter 6. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

Organopalladium intermediates are also involved in the synthesis of ketones and other carbonyl compounds. These reactions involve acylpalladium intermediates, which can be made from acyl halides or by reaction of an organopalladium species with carbon monoxide. A second organic group, usually arising from any organometallic reagent, can then form a ketone. Alternatively, the acylpalladium intermediate may react with nucleophilic solvents such as alcohols to form esters. 

The readers will be aware of the basic principles of bonding in organometallic complexes, especially the bonding of alkenes and carbon monoxide, and this will not be dealt with here. We will say a few words about ligands containing phosphorus as the donor element. 

Coupling of organometallic reagents with hahdes in a carbon monoxide atmosphere leads to ketones by incorporation of a carbonylation step.147 148 These reactions involved a migration of one of the organic subsituents to the carbonyl carbon, followed by reductive elimination. These reactions can be carried out with stannanes149 or boronic acids150 151 as the nucleophilic component. 

As with metal-carbon monoxide bonds, the MP2/6-3IG model does not lead to results of the same calibre as those from density functional models (except local density models). The model actually shows the opposite behavior as 6-3IG, in that bond lengths are consistently shorter than experimental values, sometimes significantly so. In view of its poor performance and the considerable cost of MP2 models (relative to density functional models), there seems little reason to employ them for structural investigations on organometallics. 

Organometallic complexes frequently are susceptible to nucleophilic attack by an external reagent. In some instances the attack takes place on the metal center (see substitution reactions, page 686). while in others it occurs on a bound ligand. Already in this chapter we have seen many instances in which coordinated carbon monoxide undergoes nucleophilic attack. Examples include reactions with H to produce a formyl complex (Eq. 15.19). with R to form an acyl complex (Eq. 15.49). and with OH to give a hydroxycarbonyl complex (Eq. 15.21). 

Organom etallic Compounds. Organometallic complexes of platinum are usually more stable than palladium complexes. Carbon monoxide complexes of platinum are formed more readily than with palladium. Mononuclear and polynuclear complexes in oxidation states 0 to +2 exist such as... 

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