Nickel halo complexes

Table 96 Bond Distances and Angles in some Halo Complexes of Nickel(II) ...

A variety of nickel(O) complexes, when treated tvith allyhc electrophiles, afford jt-allyl complexes (see also Houben-Weyl, Vol. E18, pp 64 and 76).l In early studies, tetracarbon-ylnickel(O) was widely employed. However, otving to its extreme toxicity, it is now rarely used. Direct treatment of bis(T] -cycloocta-l,5-diene)nickel(0) (2) with allyl halides such as 20 is now the method of choice for the stoichiometric preparation of nickel-3t-allyl complexes. In the absence of strong donor ligands such as phosphines, halo-bridged dimers (e.g., 21) are t5T3ically obtained (Scheme 12). In the presence of phosphines, monomeric species such as 22 may be obtained. Other less-electrophilic allylic substrates such as allylic ethers and allylic alcohols also serve as precursors to nickel-Jt-allyl complexes in cataljdic procedures. However, these precursors are less widely used than allyl halides in the stoichiometric preparation of the Jt-allyl complexes. 

Finally, it is also well known that the bis(halo)bis (phosphine)nickel(II) complexes present equilibria in solution between the diamagnetic and paramagnetic stereospinomers. 

An unusual elimination reaction described as a linear alkyne trimerization takes place when solid samples of nickel (halo)acetylide complexes containing PMc3 are heated (Equation (80)). These compounds have been used in the synthesis of complex polyunsaturated hydrocarbon molecules (Scheme 46). ... 

Aryl halides may also be electrochemically coupled with a-halo esters, when catalyzed by nickel complexes (equation 146)833,834. 

Syntheses of discrete cationic a-diimine complexes of palladium and nickel are well developed. DAB complexes containing halo ligands can be reacted with a variety of salts of noncoordinating anions to yield cationic organometallic species. For instance, the reaction in eq 2 illustrates chloride abstraction with NaBAF. " The resulting complexes are... 

This reaction was first reported by Mori and Ban in 1976. It is a synthesis of indole derivatives by an intramolecular Heck Reaction of o-halo-fV-aUylanilines catalyzed by a low-valent metal complex and is known as the Mori-Ban indole synthesis. In this reaction, the low-valent metal can be nickel or palladium, but the ortho halogen must be bromine or iodine. Often the o-iodo-fV-allylaniline is more reactive than corresponding o-bromo substrate. In addition, it has been found that the catalyst can be deactivated under the reaction conditions, thus aperiodic provision of fresh catalyst normally gives higher overall yields than that using the same total amount of catalyst at once. ... 

Electrophilic nickel(II) salts, when treated with allyl organometallics, afford nickel(ll)- t-allyl complexes. Rather than producing dimeric halo-bridged complexes as observed by the oxidative addition route (Section 1.1.2.1), monomeric bis(aUyl) complexes are instead obtained (Scheme 13). Although both the method described in Section 1.1.2.1 and this method afford structurally different jt-allyl complexes, disproportionation may, in some instances, allow the interconversion of a bis(allyl) complex 23 and the corresponding halo-bridged dimer 24 if a dihalonickel species is present. 

A detailed mechanistic study of the coupling of jt-allyl complexes and organic halides has been carried out.l A mechanism involving the establishment of a pre-equilibrium between bis(allyl)nickel complexes and monoallyl halo-bridged dimers is proposed. A single-electron-transfer mechanism then initiates cross coupling via nickel(I) intermediates. 

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