Transition Metals, Carbonyls

Carbon monoxide is an extremely weak Lewis base towards conventional Lewis acids. It does not complex with the boron trihalides, although it does yield a weak adduct H3B CO with diborane. On the other hand it forms numerous complexes with transition elements. The source of this difference is that in the latter complexes not only is the weakly donating a-orbital of CO involved, but also the 7C orbitals which can function as acceptors. The conventional explanation is that a synergic effect exists in which the n interaction removes electron density from the metal, allowing a donation from the ligand to be enhanced. 

Vanadium hexacarbonyl is monomeric and is the only simple carbonyl which is paramagnetic at room temperature (/Xgff= 1.81 B.M.). Steric effects have been held responsible, but this explanation is hard to believe, especially as a robust tetrahedral cluster (Ph3PAu)3V(CO)5, in which vanadium is eight-coordinate, 

First transition series V(CO), Blue-green pyrophoric crystals (/ieff 1.81 B.M). Octahedral (Oj ) Cr(CO), Colourless sublimes m.p. 150°C Air stable Octahedral (Oh) Mn,(CO),o Golden-yellow m.p. 154°C Air stable (D,h) Fe(CO)3 Yellow liquid b.p. 103°C Air stable Trigonal bipyramid (D h) Fe,(CO), Golden plates Air stable Fe3(CO), Black solid Co,(CO), Orange-red crystals m.p. 51°C dec. Air sensitive Co (CO), Black solid CoJCO), Black solid Ni(CO) Colourless liquid b.p. 42°C Air sensitive toxic, carcinogenic Tetrahedral (T d 

In general carbon monoxide can be considered to be a two electron donor as far as electron counting is concerned (see p. 191). It can act as a terminal ligand or as a bridge between two (jU -CO) or three ii -CO) metal atoms. In nearly all known examples of bridge formation, the metal atoms are within bonding distance of 

Terminal Doubly Semi-bridging (V / Triply bridging 

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Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Garbonylation of Olefins. The carbonylation of olefins is a process of immense industrial importance. The process includes hydroformylation and hydrosdylation of an olefin. The hydroformylation reaction, or oxo process (qv), leads to the formation of aldehydes (qv) from olefins, carbon monoxide, hydrogen, and a transition-metal carbonyl. The hydro sdylation reaction involves addition of a sdane to an olefin (126,127). One of the most important processes in the carbonylation of olefins uses Co2(CO)g or its derivatives with phosphoms ligands as a catalyst. Propionaldehyde (128) and butyraldehyde (qv) (129) are synthesized industrially according to the following equation ... 

The primary photochemical of transition metal carbonyls [Me(CO)n] involves the scission of carbon monoxide (CO) and the formation of coordinated unsaturated species ... 

Strohmeier and Hartmann [14] first reported in 1964 the photoinitiation of polymerization of ethyl acrylate by several transition metal carbonyls in the presence of CCI4. Vinyl chloride has also been polymerized in a similar manner [15,16] No detailed photoinitiation mechanisms were discussed, but it seems most likely that photoinitiation proceeds by the route shown in reaction Scheme (9). 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

Vibrational spectra of transition metal carbonyl compounds. L. M. Haines and M. H. B. Stiddard, Adv, Inorg. Chem. Radiochem., 1969,12, 53-133 (340). 

Matrix isolation studies on transition metal carbonyls and related species. J. K. Burdett, Coord. Chem. Rev., 1978, 27,1-58 (212). 

Thermochemical studies of organo-transition metal carbonyls and related compounds. J. A. Connor, Top. Curr. Chem., 1977, 71, 72-110 (122). 

Ion pairing effects on transition metal carbonyl anions. M. Y. Darensbourg, Prog. Inorg. Chem., 1985,33,221(138). 

The simplest transition metal carbonyls are mononuclear of the type M(CO)x, in other words those with only one metal atom. They are hydrophobic but soluble to some extent in nonpolar liquids, such as n-butane or propane. The dinuclear carbonyls are more complex but have the same general characteristics as the mononuclear carbonyls. The carbonyls, which are or could be used in CVD, are listed in Table 3.4 with some of their properties. 

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. 

Anions derived from transition-metal carbonyls (see also 8.3.3.1) react with Cu-or Ag-halogcn bonds e.g., triarsMBr (M = Cu, Ag) reacts with univalent or divalent carbonyl anions ... 

Whereas Cu and Ag form complexes with derivatives of transition-metal carbonyls in which the Cu or Ag are 4 coordinated in both the reactant and in the final product, many analogous Au complexes exhibit only 2 coordination. For example, the simple Au complex Ph3PAuCI reacts with transition-metal carbonyl anions in THF to give complexes with transition-metal to Au bonds, e.g. ... 

Anions of transition-metal carbonyls are prepared by reducing the carbonyl and react with derivatives of group-IIB metals to yield mono-, di- and even trisubstituted derivatives ... 

Mercurydl) halides displace Me3Sn groups from a transition-metal carbonyl derivative ... 

Because transition-metal carbonyls react with Zn, Cd and Hg to form compounds containing metal-mctal bonds (see 8.3.3.4), the derivatives can be prepared by combining reactants under carbonyl-forming conditions ... 

Reactions of tetrathiastannolanes bearing bulky substituents with groups 6 and 8 transition metal carbonyls have been reported. Tbt(Tip)SnS4 reacts with W(C0)5(THF) in THF at room temperature to give Tbt(Tip)SnS4-W (CO)s in 40% yield (Scheme 64) [144]. 

Heterogeneous catalysis by metals has been of long-standing interest, with bimetallic catalysts a particular focus.Transition metal carbonyls have... 

It has been noted (Section II,B,1) that reactions between transition metal carbonyl anions and silicon halides often fail to produce species containing silicon-transition metal bonds, and that such failure has been ascribed to nucleophilic attack by carbonyl oxygen. It is therefore interesting that compounds containing Si—O—C—transition metal linkages have recently been isolated from such reactions [Eqs. (105) (R = Me, Ph) 183) and (106)... 

Application of small metal particles has attracted the attention of the scientists for a long time. As early as in the seventies Turkevich already prepared mono-dispersed gold particles [19], and later, using molecular transition metal carbonyl clusters [20], the importance of small nanoparticles increased considerably. One of the crucial points is whether turnover frequency measured for a given catalytic reaction increases or decreases as the particle size is diminished. 

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