Hydride complexes nickel

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

A mononuclear nickel hydride complex with three N-heterocyclic carbene ligands has been reported the compound was formed by oxidative addition of an imidazolium salt to the Ni(0) bis(carbene) complex [19]. The hydride signal of this nickel(II) complex appears at -15 ppm. 

Similarly, condensation of a mixture of CsHe and C3D6 with nickel vapor resulted in isotopic scrambling. The intraligand isomerizations may be explained in terms of a 1,3-hydrogen shift via a nf-allyl nickel hydride complex in equilibrium with a ir-propene complex, e.g.,... 

The addition-elimination mechanism, however, is strongly preferred for monohydride systems such as [HCo(CO)4]187 and the Vaska complex193,194 promoting extensive isomerization. Hydroformylation of 2-pentenes in the presence of [HCo(CO)4], for instance, yields mainly the nonbranched aldehyde resulting from double-bond migration.195 Nickel hydride complexes are one of the most active... 

The isomerization of the internal olefin 3PN to the terminal olefin 4PN is a critical step in the hydrocyanation of 3PN to ADN [Eqs. (9) and (10)]. Unfortunately, there is a loss in yield because the undesirable conjugated isomer 2PN is also produced. Observations discussed below have led us to the belief that cationic nickel-hydride complexes, HNiL4, may be important in the isomerization process. 

A nickel hydride complex, NiHCl(diphenylphosphinoethane), catalyses the tandem isomerization-aldolization reaction of allylic alcohols with aldehydes.156 The atom- (g) efficient process proceeds at or below ambient temperature with low catalyst loading, and works well even for bulky aldehydes. Magnesium bromide acts as a co-catalyst, and mechanistic investigations suggest that a free enol is formed, which then adds to the aldehyde in a hydroxyl-carbonyl-ene -type reaction. 

Under the reaction conditions the precursor complex probably generates a nickel-hydride species, which then initiates the oligomerization reaction. Evidence for this comes from the studies on the reactions of 7.17. As shown by 7.6, on reaction with ethylene 7.17 eliminates styrene and produces a nickel-hydride complex. A model catalytic intermediate 7.18 has been characterized by single-crystal X-ray studies. Complex 7.18 reacts with ethylene to give a nickel-ethyl species in a reversible manner. This is shown by reaction 7.7. Reactions 7.6 and 7.7 are strong evidence for the involvement of a nickel-hydride catalytic intermediate. 

The nickel-hydride complex that acts as a precatalyst for this isomerization reaction is thought to be the cationic part of 7.52. The evidence for the existence and participation of a cationic species such as 7.52 comes from multi-nuclear NMR and IR data. An equilibrium as shown by 7.23 exists, and the cation [HNiL4]+ is the dominant precatalyst for the isomerization reaction. The cation is an 18-electron complex. It undergoes ligand dissociation to give 7.53 before alkene coordination takes place. A ligand dissociated species such as 7.53 with L = p-tolylphosphite has been observed spectroscopically at low temperatures. 

Nickel hydride complexes had been considered to be very unstable before the discovery of several stable hydride complexes of the type NiHX(PR3)2... 

In view of the chemical reactivity of the tertiary phosphine and nickel hydride complexes to air, all manipulations should be carried out in a nitrogen or argon atmosphere.6 All solvents should be distilled under a nitrogen or argon atmosphere. 

The isomerization of 3PN to 4PN (equation 7) is catalyzed by a cationic nickel-hydride complex generated by removal of cyanide by a Lewis acid promoter (equation 10). 

The catalytically active species involved in the oligomerization reactions are probably nickel-hydride complexes, as indicated by their activity in carbon-carbon bond formations (oligomerization) and carbon-carbon double bond shifts (isomerization). The proposed mechanism of these biphasic reactions is analogous to the reactions occurring under one-phase conditions. ... 

The mechanism of ethylene oligomerization by SHOP and related catalysts, such as phosphinophenolate complexes, has been the subject of intense investigation (see COMC (1982), Chapter 52, references cited by Heinicke et al. and Pietsch et air It is generally agreed that the actual catalytic species are nickel hydride complexes, generated by ethylene insertion into the Ni-aryl bond of the precursor followed by /3-H elimination reaction (Scheme 50). Styrene or styrene derivatives can be detected in the reaction medium as a product of this activation process. In the case of the salicylaldiminate and anilinotropolone catalysts, styrene elimination is not required,... 

A bidentate cyclopentadienyl-functionalized NHC complex of nickel(II) catalyses hydrosilylation of aldehydes, allowing quantitative reduction in 5 min at 25 A transient nickel hydride complex, ( Cp-NHC)NiH, is implicated as the active species. 

A square-planar nickel hydride complex is suggested as the catalytic species [589]. In the first step, the nickel hydride catalyst adds across the double bond of propylene to give two intermediates, namely, a propyl nickel and isopropyl nickel complex. Both of these intermediates can react further with propylene by insertion of the double bond into the nickel-carbon bond, resulting in formation of four more intermediates. ( -Elimination of nickel hydride from these intermediates produces the possible products of propylene dimerization, namely, 4-methyl-1-pen-tene, cis- and trans-4-methyl-2-pentene, 2,3-dimethyl-l-butene, n-hexene, 2-hexene, and 2-methyl-l-pentene. Terminal unbranched olefins are rapidly isomerized under the influence of catalyst by a process of repeated nickel hydride addition and elimination to the internal olefins. Therefore, under ordinary reaction conditions the yield of 4-methyl-l-pentene is low. 

Petrushanskaya, N. V., Kurapova, A. I., Rodionov, N. M., and Feldbljmm, V Sh., Olefin dimerization under the influence of nickel hydride complexes, Zhur. Org. Khim., 10, 1402, 1974 (Russian). 

Another interesting and related reduction reaction of CO2 is its reaction with catecholborane (HBcat) catalyzed by a nickel hydride complex, which was recently reported by Guan and coworkers [24]. In collaboration with Wang, they also theoretically investigated the reaction mechanism [25]. Scheme 6.7 shows the reaction mechanism supported by the theoretical study. The catalytic cycle consists of three major reduction reactions (i) CO2 reacts with the first... 

As mentioned earlier, the first reaction in the second stage is the isomerization of 3PN to 4PN, i.e., reaction 5.6.4. The general mechanism of alkene isomerization reaction involves insertion of alkene into the metal carbon bond followed by /3-elimination. The mechanism in this particular case is no different and is shown in Figure 5.10. The following points need attention. The nickel-hydride complex that acts as a precatalyst for this isomerization reaction is thought to be the cationic part of 5.64. 

The liquid-phase industrial process for the dimerization of propylene is called the dimersol process. In this process, a Zeigler-type catalyst is generated in situ by the treatment of a nickel salt with trialkyl aluminum. The different isomers of C. alkenes that are formed can be explained by referring to Figure 7.1. A nickel hydride complex 7.1 initiates the dimerization reaction. Complexes 1.1 and 73 are formed by the insertion of the first propylene into the Ni-H bond in anti-Markovnikov and Markovnikov maimer, respectively. 

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