Palladium derivatives, zerovalent

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). 

Chemistry of zerovalent nickel, palladium and platinum derivatives... 

Chemistry of Zerovalent Nickel, Palladium, and Platinum Derivatives... 

The chemistry of the zerovalent state in nickel, palladium, and platinum compounds is reviewed. After a historical introduction in which the development of this chemistry is analyzed in terms of the current theory of the stabilization of low valency states, the most interesting classes of zerovalent compounds are described. The stability and properties of these compounds are discussed and related to the nature of the ligands and the coordinated metal. The catalytic properties of these zerovalent derivatives toward olefins, diolefins, and acetylenes are considered in connection with the facility of ligands exchange, the variation of coordination number, and the stereochemistry. A discussion of the % bond is reported. 

There are different methods for preparation of the zerovalent nickel, palladium, and platinum derivatives. Generally, these methods are similar to those usually employed in the chemistry of the low oxidation states. The more commonly used are the following ... 

By Replacement of Zerovalent Isocyanide Derivatives. This method was described for preparing the tris and tetrakis palladium (0) derivatives with tertiary aromatic phosphines and phosphites 129). It might be extended to nickel. 

The zerovalent cyanometallates are compounds with strong reducing properties, and they are known to reduce water. The stability of these complex cyanides decreases with increasing atomic number. The nickel compound is stable in vacuo) up to 160° C. the palladium compound decomposes slowly in vacuo) at ordinary temperatures, while the platinum derivative could not be isolated at all. 

In conclusion, the zerovalent nickel, palladium, and platinum derivatives still offer a fertile field for new and extensive researches. 

It has been shown that electron-rich cyclopropanes are able to displace ethylene from dichloro(ethylene)platinum to yield four-membered metallocyclic complexes (cf. equation 37). On the other hand 1,1,2,2-tetracyanocyclopropane (171) reacts under mild conditions with zerovalent platinum and palladium complexes of the type Pt(PPh3)2 (C2H4) or ML (n = 3, 4 M = Pd or Pt L = phosphines or triphenylarsines) to give metallocyclobutane derivatives (172) (equation 118) . 

Phase-transfer catalysis is a useful procedure for a variety of interesting metal-catalyzed reactions. - However, only one example of this approach has been reported for the synthesis of diynes by the sp-sp carbon coupling reaction. Thus vinylic dibromides derived from aromatic aldehydes have been shown to react with carbon monoxide, in the presence of zerovalent palladium compounds as the metal catalyst, and under phase-transfer conditions in a two-phase system (benzene, 5 M NaOH), to give the corresponding diynes in reasonable yields (equations 21 and 22). ... 

Evidence for the intermediate formation of nitrone species during the carbonylation of nitroarenes in e s-cyclooctene as solvent catalysed by Ru3(CO)i2 have been obtained [14], Moreover, zerovalent palladium species with nitrogen donor ligands have been shown to be active catalysts in the reductive carbonylation of organic nitro derivatives [41]. The hypothesis that an intermediate having the olefinic double bond coordinated to the metal is formed during the catalytic cycle is supported by the steric effect that has been observed in the case of p,p -dimethyl-o-nitrostyrene (7f) as substrate. Moreover, such an intermediate could explain why a pentaatomic indole nucleus is preferentially formed, even when a conjugated double bond is present in the olefinic chain ... 

Allyl Complexes and Complexes Derived from Mono-ol ns Zerovalent Nickle, Platinum, and Palladium.— A series of complexes Ni(ol) (x=l—3, ol=alkyl, chloro, or fluoro olefin) has been detected by matrix condensation of nickel atoms with the olefin the optical spectra show a correlation between the transition energies of the Ni(ol) species and the 7r -orbital energy of... 

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