Nickel 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). 

A3-Pyrrolinones have also been obtained from metal-mediated cyclooligomerization processes in which concomitant hydrolytic or carbonyl insertion occurs. For example, tert-butyl isocyanide is converted in aqueous methanol by zerovalent nickel compounds e.g., Ni(t-BuNC)4, Ni(CO)4, into a di(alkylamino)-A3-pyrrolinone in moderate yield (Scheme 34). The reaction takes a different course in anhydrous methanol in which a di-tert-butylamino)ethylene derivative is formed, albeit in poor yield (Scheme 34).62... 

This potassium salt, K4Ni2(CN)6, may be further reduced by potassium in liquid ammonia to yield a yellow substance, K4Ni(CN)4. This has nickel in the zero-valent state and is thus comparable to the metal carbonyls, Fe(CO)5 and Ni(CO)4 (p. 157), to cobalt nitrosyl carbonyl Co(CO)4NO, and to the metal ammoniates Ca(NH3)6 and Pt(NH3)2. However, K4Ni(CN)4, and the closely related acetylene derivative, K4Ni(C=CH)4, are especially unusual, for in them, the zero-valent metal has been incorporated into an anion, whereas in the carbonyls and metal ammoniates, the zerovalent metals are present as uncharged species. 

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 ... 

Direct Synthesis from the Metals. The first zerovalent compound prepared, namely Ni(CO)4, was first obtained by this method—i.e., from active nickel and carbon monoxide at ordinary temperature and pressure mi)- Though not largely applicable, this method can be used in some other cases. K4[Ni(CN)4] could be prepared from metallic nickel and potassium cyanide (205), while some chloro- (156), bromo- 125) and fluorophosphine (118) and some tertiary diphosphine (44) could react with metallic nickel to give the tetracoordinated nickel(O) derivatives. 

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 nature of platinum zerovalent derivatives was questioned by Chopoorian et al. (55) who considered them platinum(II) dihydrides. A new careful investigation (47, 133)j however, has confirmed that they are certainly zerovalent compounds. The nickel(0) derivatives with tertiary monophosphines can be obtained as well from NiBr2L2 and sodium tetrahydridoborate or ethanolic potassium hydroxide in the presence of an excess of the ligand 185) j while those with phosphites were prepared using potassium graphite 110) (CsK) as the reducing agent. By reduction with alkylmetals 199), zerovalent tetrasubstituted nickel (0) compounds with tertiary phosphine, arsines, and stibines have been obtained. 

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. 

In the reaction with nitrosobenzene, azoxybenzene and Ni(PhN02>2 are formed. The nickel complex is an interesting example of a" coordination compound of a nitro derivative with a zerovalent metal. The X-ray analysis of this compound has been undertaken 383, but the final structure has not yet been reported. The reaction with Bu NC can be carried out catalytical 1y. Thus a mixture of PhNO and Bu NC in benzene, in the presence of a catalytic amount of NiCBu NO at reflux, produces Bu NC0( SOX based on PhNO) along with PhN=C=NBu, PhN=NPh and [Pg.105]

The reaction of butadiene with alkynes to give cyclodecatrienes is catalysed by zerovalent nickel complexes. Evidence that these reactions proceed in a stepwise manner and involve an aw-bisallyl-Cg nickel complex has now been obtained. Thus addition of Et02CC CC02Et to the aco-octadienediylnickel complex (40) at — 30 °C gives a yellow complex (41) which on treatment with carbon monoxide gives the cyclodecatriene derivative (42) (Scheme 10). ... 

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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