Group VIII Metal Complexes

Alkyl, allyl, and aryl bromides are dehalogenated mainly with the formation of R R dimers in the presence of polypyridyl complexes of the metals of Group VIII. It has been demonstrated that the complexes [Co(bpy)3] + 203-204 [Ni(bpy)3]2+,205 and [Ni(phen)3]2+206 catalyze the reductive dimerization of allyl and alkyl bromides in organic 203 205 206 and aqueous micellar 204 solution. 

This preparation is an illustration of the hydroformylation of olefins (oxo synthesis). The reaction occurs in the presence of soluble catalytic complexes containing metals of Group VIII of the periodic system. Although the metal originally used by Roelen and still largely used in the industry for the production of aliphatic aldehydes and alcohols is cobalt, the most active and selective catalysts are rhodium-containing compounds. The catalytic activity of the other Group VIII metals is in... 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

The hydrogenation of 2-butyne has been studied over the other metals of Group VIII and over copper (84, 95) using a static system and alumina-supported catalysts in the temperature range 100 to 200°. Under these conditions more complex distributions of products have been observed than was the case for palladium at room temperature. 

Chalk and Harrod364 have investigated the mechanism of this hydrosilation of olefins under catalysis by complexes of metals of Group VIII. 

Group 15 complexes/group VIII metal surfaces... 

Because of the limited tendency for complexes to react with hydrogen, this reaction serves as an excellent example of the relative abilities of various metals in Group VIII to undergo oxidative addition and illustrates the influence of ligands on the reactivity of d complexes. For example, the iridium complex Ir 0)(PPh3)2 readily reacts with hydrogen at atmospheric pressure, but the isostructural rhodium complex does not form a... 

This reaction is catalysed by metals of Group VIII and their complexes. The preparation of triethylpropanoyloxysilane [271] (95% b.p. 188°C) from trieth-ylsilane apd propanoic acid (30 min at 70 °C) depends on a small amount of tris(triphenylphosphine)-rhodium(I)chloride as catalyst. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

The catalysts used are themselves complexes produced by interaction of alkyls of metals in Groups l-IIl of the Periodic Table with halides and other derivatives of Groups IV-VIII metals. Although soluble co-ordination catalysts are known, those used for the manufacture of stereoregular polymers are usually solid or adsorbed on solid particles. 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

The formation of TMM complex from Group VIII transition metal such as Ir, Ru, and Os from precursors derived from (1) has been reported M.D. Jones, R. D.W. Kemmitt,/. Chem. Soc., Chem. Commun., 1985, 811-812. 

The Mechanism of Dehydration of Alcohols over Alumina Catalysts Herman Pines and Joost Manassen Complex Adsorption in Hydrogen Exchange on Group VIII Transition Metal Catalysts... 

The dioxygen ligand in mononuclear group VIII transition metal complexes. J. S. Valentine, Chem. Rev., 1973, 73, 235-245 (101). 

The most important metals for catalysis are those of the groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonally dose-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe). Figure 5.1 shows the unit cell for each of these structures. Note that the unit cells contain 4, 2, and 6 atoms for the fee, bcc, and hep structure, respectively. Many other structures, however, exist when considering more complex materials such as oxides, sulfides etc, which we shall not treat here. Before discussing the surfaces that the metals expose, we mention a few general properties. 

A full discussion of the reactions of molecular oxygen involved in dioxygen adducts would require a separate review. Moreover, the reactions of co-ordinated molecular oxygen are discussed elsewhere l-8) dioxygen adducts of biological systems (7) and synthetic group VIII metal complexes 1,3) being of particular interest. 

Bayer and Schretzmann 25) came to the conclusion that reversible oxygenation is a characteristic property of group VIII metals. However, work has shown that the cadmium complex CdEt2 can take up dioxygen reversibly in the ratio 1 2 (Cd O2). But it was found that the oxygenated complex (II(P) or 11(G) orientation) can undergo spontaneous catalytic oxidation to form bis(ethylperoxy) cadmium ... 

Group VIII Transition Metal Dithiocarbamato Complexes.97... 

A variety of Group VIII transition metal phosphine complexes are shown to be active catalysts for hydrogenation of aliphatic nitro compounds. However, chiral phosphines have been found to be noneffective to induce asymmetric induction.110... 

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