Metal Carbonyls and Their Derivatives

The free energies of activation for rotational interconversion of anti and gauche isomers in the series [(tj -CsH5)M(CO)3]2 (Fig. 3 L = rf—C5H5, L = CO) increases from Cr (51 kJ mole ) through Mo (63 kJ mole ) to W (68 kJ mole ) (2) this is the inverse order to that for the M—distances, indicating particularly that the long Cr—Cr bond is flexible and torsion about it is more ready. Indeed, above ca. 274 K, a 

On the other hand, in one example, [Rh P(OMe)3 4]2 (55), steric inhibition to intramolecular exchange is so great that, at 373 K, a specific intermolecular exchange of only the equatorial ligands is observed [Fig. 3 L = L = P(OMe)3]. 

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Photochemical substitutions on metal carbonyls and their derivatives. W. Strohmeier, Angew. Chem., Int. Ed. Engl., 1964, 3, 730-737 (68). 

Caution. Since metal carbonyls and their derivatives are toxic volatile compounds, all operations must be performed in an efficient hood. 

The early workers in coordination chemistry were more interested in the theory of bonding and structure than in any practical usefulness which the compounds might have. In more recent times, however, applications have developed. Perhaps the most important of these is in catalysis, especially for hydrogenation and the activation of carbon-hydrogen bonds. Metal carbonyls and their derivatives have played a large part in this application, as well as in carbonylation reactions such as the recently developed process for converting methanol to acetic acid 42... 

The highly covalent nature of transition metal carbonyls and their derivatives leads to the 18-electron rule being closely followed. The mononuclear species Ni(CO)4, Fe(CO)5, Ru(CO)5, Os(CO)5, Cr(CO)6, Mo(CO)6 and W(CO)6 obey this well and, if the formalized rules of electron counting are applied, so do the metal—metal bonded and carbonyl bridged species. Such compounds are therefore coordinately saturated and the normal (but by no means unique) mode of substitution is dissociative (a 16-electron valence shell being less difficult to achieve than one with 20 electrons).94... 

Approximately 25 years after Mond and co-workers had prepared the first carbonyl of cobalt, Co2(CO)8 (78), Hieber obtained HCo(CO)4 by the acidification of salts containing [Co(CO)4] (79), one of the first established carbonylmetallates (80). In 1941, Behrens, a student of Hieber, initiated his important and extensive investigations on the reduction of metal carbonyls and their derivatives by alkali and alkaline earth metals in liquid ammonia (81). At this time he established that various salts of [Co(CO)4]-, including Na[Co(CO)4] and K[Co(CO)4], could be obtained from the neutralization of HCo(CO)4 or the reduction of Cd[Co(CO)4]2 by sodium or potassium in liquid ammonia (4). On the basis of Behrens pioneering studies, we were... 

Transition metal carbonyls and their derivatives are remarkably effective and varied in their ability to catalyze reactions between unsaturated molecules (e.g., CO and olefinic compounds) or between certain saturated and unsaturated molecules (e.g., olefins and H2 or H20). The carbonyl derivatives of cobalt are particularly active catalysts for such reactions and have been put to use in the industrial synthesis of higher aliphatic alcohols. In fact, much of the growth in knowledge concerning catalysis by metal carbonyls has been stimulated by the industrial importance of the Fischer-Tropsch synthesis, and by the economically less important, but chemically more tractable, hydroformylation reaction. 

The most characteristic feature in the mass spectra of the metal carbonyls and their derivatives is the successive loss of CO groups to give a series of... 

Based on the foregoing experimental results, the versatility of metal carbonyls and their derivatives in their reactions with liquid NH3 may be summarized as follows (/) substitution of CO or other ligands by NH3 without change in the oxidation number of the transition metal in question (2) conversion of covalent carbonyl complexes into ionic compounds by addition of NH3 molecules (3) base reactions" in which the transition metal is reduced to a carbonyl metalate with complementary oxidation of a CO ligand to CO(NH2)2 (4) valence disproportionations with... 

For case (3) complexes, examples of which include many metal carbonyls and their derivatives (Table 8.8.3), the l/2g orbitals are strongly bonding due to back donation and the 2eg orbitals are strongly antibonding. Thus, while it is still imperative not to have electrons occupying the 2eg orbitals, it is equally... 

Early in the development of the chemistry of the metal carbonyls and their derivatives, Cotton et al. (42) concluded that the number of infrared-active carbonyl stretching vibrations of a complex is determined by the idealized local symmetry of the terminal carbonyl groups. Although exceptions to this generalization are to be found (50, 60, 117, 134), its utility is now widespread (5, 17, 60, 116, 119). A consequence of this generalization is that the isomeric form of a complex may be determined... 

Zero- and lower-valent Mo chemistry is dominated by organometallic species, mostly metal carbonyls and their derivatives. A wide variety of other complexes, including a-aryl, cr-alkynyl,... 

There are several recent reviews of the photochemistry of transition metal complexes 3,4,5,29,30,45,466,514,518,580)> the most comprehensive being 4>. However, these papers deal mainly with ionic coordination compounds containing inorganic ligands only one 466> is devoted to photochemical substitution reactions of metal carbonyls and their derivatives, while in another 4> this subject is discussed in a short paragraph. 

The metal carbonyls and their derivatives are discussed in Section II, followed by the hydrocarbon-metal complexes, classified by the bonding capacity of the organic ligand, in Sections III—IX. When two such hydrocarbons are present the compound is included in the section dealing with the ligand of higher hapticity. Sandwich complexes, such as the metallocenes and [M(>7-arene)2], are described in Section X as a special, related group of molecules. 

Wolkenstein s bond polarizability approach to Raman intensities has been applied to the terminal carbonal stretching vibrations of M(CO)g (M = Cr, Mo, or W) and the general intensity formulae given. The model is valuable in providing an insight as to why the totally symmetric stretching modes of metal carbonyls and their derivatives frequently give rise to weak bands in the Raman effect. ... 

Metal carbonyls are compounds of transition metals with carbon monoxide ligands. They find applications in many catalytic processes and in organic synthesis. Thus, research on preparations, structures, and applications of metal carbonyls and their derivatives has been intensive for several years. The first metal carbonyl, Ni(CO)4, was prepared by A. Mond, G. Langer, and F. Quincke in 1890 by the reaction of metallic nickel with carbon monoxide. It found application immediately it has been used for industrial preparation of pure nickel. Many other carbonyls were synthesized shortly thereafter. 

Photochemical conditions are widely used in synthetic organometallic studies, and there is a steadily increasing number of quantitative studies concerned with quantum yields and elucidation of the photochemical mechanism. Most of the systematic work has been done on metal carbonyls and their derivatives, and the following discussion will be limited to this class of compounds. 

Stereochemical Nonrigidity in Metal Carbonyls and Their Derivatives... 

New chemistry of metal carbonyls and their derivatives is summarized below. However, as in previous Reports many carbonylmetal complexes are reviewed in other chapters. Herein we highlight significant results of general interest. A Bibliography at the end of the chapter lists papers which were noted during our survey of the literature. In this introductory section the reader s attention is drawn to some novel discoveries related to new carbonylmetal chemistry. 

Single-crystal X-ray and neutron diffraction studies on metal carbonyls and their derivatives are summarized in the last chapter of this Report. Here we mention some results of special interest. 

Hence it is more appropriate to view noble-metal carbonyls and their derivatives as coordination complexes of CO rather than as organometallic compounds. A comparison to metal cyanide complexes is far more appropriate, in particular since [Au(CO)2]+ and [Au(CN)2] as well as [Pt(CO)4]2+ and [Pt(CN)4]2- are isoelec-tronic and isostructural pairs. Strong bonding similarities have been established for the first pair (9). It is unlikely that cationic metal-carbonyl complexes will ever form so many different species as the cyanide complexes, which are known for most transition metals (86). With useful and facile synthetic routes available, further examples of cationic metal carbonyls with similar bonding and spectroscopic features as described here should be prepared and characterised in the future. 

Most of the discussion in this chapter deals with metal complexes of the Werner type. These complexes, as we have seen, are often anionic or cationic species, and their salts are water-soluble. A quite different but related class of compounds are the metal carbonyls and their derivatives. These materials are much more covalent in character and dissolve preferentially in nonpolar solvents. The metal carbonyls have been known since 1888 when Ni(CO)4 was discovered by Mond. However only in the last... 

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