Nickel complexes 5-coordinate

In the first reported examples of the asymmetric Grignard cross-coupling, a nickel complex coordinated with (-)-DIOP (1) was used as catalyst [6,7]. Reaction of 1-phenylethyl (2) and 2-butyl (3) Grignard reagents with vinyl chlo-... 

Palladium as well as nickel complexes coordinated with chiral phosphine ligands, especially those containing a dialkylamino side chain, have been used for asymmetric cross-coupling. 

Table 3 summarizes the epimerization carried out by nickel complexes coordinating monoamines (runs 44-50), ethylenediamines containing ether link-... 

The nickel complex coordinating a less alkylated diamine lost much of its ability to epimerize although it showed a high epimerizing activity to attain the equilibrium in methanol, as discussed in Sect. 3.1. Although 1,1,1 -en shows an excellent epimerizing capacity in methanolic solution, its ability vanishes in water. 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

The cocondensation of nickel atoms and CS2 at 12 K resulted in the formation of three binary, mononuclear, nickel/CS complexes, NKCSjln, n = 1-3 (145). Mixed CS2/ CS2 isotopes were used to identify the lowest stoichiometry species. An interpretation of the IR and UV-visible spectra, as well as normal-coordinate analyses (144), suggested that these species are best considered as normal 7r-complexes, with the nickel atom coordinated to the C=S bond in a manner analogous to C=C bond coordination (123). 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

---