Nickel anion complexes

Lithium salts as well as those of other alkali metals can stabilize nickel complex anions as well as promote the consumption of any formed methylacetate. 

Radical pertluoroalky lation of anilines occurs in the presence of a sufur dioxide radical anion precursor, such as Zn-S02 or sodium dithionite [154, 755], or of a nickel complex [756] (equation 134)... 

In addition to the tetrahedral and octahedral complexes mentioned above, there are two other types commonly found—the square planar and the linear. In the square planar complexes, the central atom has four near neighbors at the corners of a square. The coordination number is 4, the same number as in the tetrahedral complexes. An example of a square planar complex is the complex nickel cyanide anion, Ni(CN)4-2. 

Stable enolates such as diethyl malonate anions react with allyl sulfones (or acetates) in the presence of nickel complexes to give a mixture of the a- and /-product83. The regioselectivity is generally poor in the nickel-catalyzed reaction, but the molybdenum-catalyzed reaction is selective for alkylation at the more substituted allylic site, thereby creating a quaternary carbon center84. 

A further example of a reaction which may be optimised in IL/scC02 by selection of the appropriate anion for the IL is catalytic enantioselective hydrovinylation, a synthetically interesting and truly atom economic C-C bond forming reaction [77-79]. The nickel complex below has been developed by Wilke and co-workers as precursor for a highly active and enantioselective catalyst for this process. 

The purpose of this review is to provide the reader with a scenario of what can be accomplished with nickel complexes. It is useful to do this via a series of broad schemes, hypothetical in some cases, which show the main reaction patterns, without considering mechanistic details. Examples will illustrate the various processes, but extension to other substrates or to different conditions often requires use of alternative ligands or solvents, or a change from a neutral complex to a cationic or anionic species, as indicated above. For references to syntheses with nickel see Baker et al. (10). For criteria for the synthesis of coordination compounds and stability of organotransition metal complexes in general, see references (11. 12). Organometallic literature has been collected periodically by Bruce (13). 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

Aromatic halides (example 21, Table VII) also react in the presence of Ni(0) complexes in alkaline media. Complex nickel carbonyl anions, such as Ni5(CO),22, Ni6(CO)122, Ni9(CO)182 (194) are formed which, being more nucleophilic, can attack aromatic halides. 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

Ni(SSePPh2)2], and the complexes [NiCYPPh CH Ph2PY)2] (Y = S or Se) have been prepared by deprotonation of (Ph2PY)2CH2 with BuLi followed by addition of a nickel halogeno complex anion. Both complexes are planar. ... 

Nickel forms a large number of complexes with various anions (monoden-tate, bidentate, and polydentate) and many neutral ligands. The most common coordination numbers of the metal in these complexes are six and four while the metal is usually in +2 oxidation state, Ni2+. Also, some complexes of three and five coordinations exist. Several zero valent nickel complexes, such as nickel tetracarbonyl, and a number of substituted carbonyl complexes are well known. 

Alkali metal salts of such tetracyanonickelate(II) anion may be crystallized from such solutions as hydrates, K2 [Ni(CN)4 3H2O upon evaporation of the solution. In strong cyanide solution, a pentacyano complex anion, red penta-cyanonickelate(ll), [Ni(CN)5] forms. Strong acids decompose cyanonickelate salts, precipitating nickel cyanide. 

Fluoral has been condensed with an equivalent of chiral glycinate anion. The chirality stems from a chiral nickel complex with a chiral Schiff base derived from proline as hgand (Figure 5.17). (25, 35)-Difluorothreonine has thus been obtained with an excellent selectivity (de > 95%). This method also allows preparation of numerous fluoroalkyl and fluoroaryl analogues of threonine. Enantiopure difluorothreonine could also be prepared from ascorbic acid. ... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Isoelectronic with nickel carbonyl are the anions, Ni(CN)44- and Pd(CN)44-, which are obtained as their potassium salts by reduction of the corresponding cyanides of oxidation state +2 with potassium in liquid ammonia (32, 65, 186). The infrared spectrum of the nickel complex has been reported (67) to show only one band at 1985 cm-1, in the triple-bond... 

Ephraim and Muller 2 describe some results obtained with the groups — S04, — Se04, —Mo04, — W04, and —Cr04. These anions were united with the hexammino-nickel complex, and the temperature measured at which the ammonia tension is equal to atmospheric pressure. The stability of the complex is apparently independent of the atomic volumes of selenium, sulphur, chromium, molybdenum, and tungsten, but increases proportionally with the molecular volume of the corresponding trioxides. 

The formation in solution of nickel complexes with TV-substituted ethylenediamines has been studied over a long period by many authors.707 Solutions of nickel(II) complexes with TV-substituted diamines often exhibit equilibria between pseudotetrahedral and pseudo-octahedral species. These equilibria are displaced towards the pseudotetrahedral species when the temperature increases. Complexes Ni(TV,TV-Et2en)2X2691 with poorly coordinating anions are thermochromic. This behaviour has been investigated by means of calorimetric and NMR studies.708,709... 

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