Concentration nickel complexation

Pipette 25 mL nickel solution (0.01 M) into a conical flask and dilute to 100mL with de-ionised water. Add the solid indicator mixture (50mg) and 10 mL of the 1M ammonium chloride solution, and then add concentrated ammonia solution dropwise until the pH is about 7 as shown by the yellow colour of the solution. Titrate with standard (0.01 M) EDTA solution until the end point is approached, then render the solution strongly alkaline by the addition of 10 mL of concentrated ammonia solution, and continue the titration until the colour changes from yellow to violet. The pH of the final solution must be 10 at lower pH values an orange-yellow colour develops and more ammonia solution must be added until the colour is clear yellow. Nickel complexes rather slowly with EDTA, and consequently the EDTA solution must be added dropwise near the end point. 

In the same context, Kang et al. have examined several bidentate thiol derivatives as ligands in the same reaction to that described above. A nickel complex derived from the p-amino thiol depicted in Scheme 2.31 catalysed the reaction in a useful asymmetric level of 74% ee, whereas the use of the thiol phosphine ligand depicted in Scheme 2.31 gave a lower enantioselectivity of 27% ee. In this study, the authors found that the enantioselectivity was strongly dependent on the ligand concentration and on the nickel-to-ligand ratio. 

The following conclusions can be drawn (a) ir-Allylnickel compounds are probably not involved in the catalytic dimerization of cyclooctene, because the highest reaction rate occurs when only traces of these compounds can be detected further, the concentration of the new 7r-allyl-nickel compound (19) becomes significant only after the catalytic reaction has ceased, (b) The complex formed between the original 7r-allylnickel compound (11) and the Lewis acid is transformed immediately upon addition of cyclooctene to the catalytically active nickel complex or complexes. In contrast to 7r-allylnickel compounds, this species decomposes to give metallic nickel on treatment of the catalyst solution with ammonia, (c) The transformation of the catalytically active nickel complex to the more stable 7r-allylnickel complex occurs parallel with the catalytic dimerization reaction. This process is obviously of importance in stabilizing the catalyst system in the absence of reactive olefins. In... 

Into a Schlenk tube was placed Auf 1,5-cyclooctadiene)-nickeI(0) (2.6 mmol), 2,2 -bipyridyl (2.6mmol), 1,5-cyclooctadiene (0.2ml), DMF (4ml), and toluene (8 ml). The reaction mixture was heated to 80°C for 0.5 h under argon. The dibromide comonomers 623 and 634 dissolved in degassed toluene (8 ml molar ratio of dibromides to nickel complex 0.65) were added under argon to the DMF-toluene solution and the polymerization maintained at 80°C for 3 days in the dark. 2-Bromofluorene (molar ratio of dibromides to monobromide 0.1) dissolved in degassed toluene (1ml) was added and the reaction continued for 12 h. The polymers were precipitated by addition of the hot solution dropwise to an equivolume mixture of concentrated HC1, methanol, and acetone. The isolated polymers were then dissolved in toluene or dichlor-omethane and reprecipitated with methanol/acetone (1 1). The copolymers were dried at 80°C in vacuo. The isolated yields of copolymers 240a-c were 79-85%. 

The reaction of planar Ni ([14]aneN4) + represented as shown in (2.10) with a number of bidentate ligands (XY) to produce c -octahedral Ni ([14]aneN4) XY + is first-order in nickel complex and [OH ] and independent of the concentration of XY.In the preferred mechanism, the folding of the macrocycle (base-catalyzed tmns — cis isomerization) is rate determining, and this is followed by rapid coordination of XY ... 

The formation of the active catalyst can be retarded with high carbon monoxide partial pressure. High CO partial pressure leads to more CO in solution which competes with the ligand over the tricarbonyl species, Ni(C0)3, and forms the inactive nickel tetracarbonyl. The active complex stability was retained by increasing the promoter concentration. The complex formed between nickel and promoters is more stable than Ni(C0)4. In addition, promoters may impart higher electron density to the central atom and increase its nucleophilic character towards methyl iodide. 

In the case of phosphine, the active catalyst is presumably either bisphosphine dicarbonyl or the phosphine tricarbonyl complex. Kinet-ically the bis-phosphine nickel complex cannot be the predominant species. However, in the presence of very high phosphine concentration it may have an important role in the catalyst cycle. After ligand loss and methyl iodide oxidative addition, both complexes presumably give the same 5 coordinate alkyl species. 

The four-coordinate alkyl complex, LNiI(C0)CH3, may coordinate with carbon monoxide to regenerate the five coordinate alkyl species, and this leads to insertion to form Ni-acyl complex. This complex, LNil (CO)(COCH3), can be cleaved either by water yielding acetic acid or by methanol to give methyl acetate. However, in the presence of high iodide concentration formation of acetyl iodide may predominate (29). This step is reversible and can lead to decarbonylation under low carbon monoxide partial pressure. Similar decarbonylations of acyl halides by nickel complexes are known (34). 

The 5,5-CHIRAPHOS contains traces of a nickel complex (a concentrated solution appears yellow-brown) which is removed by a single recrystallization, which also increases the magnitude of the optical rotation by 3-4°. 

A secondary, more subtle, effect that can be utilized in the achievement of selectivity in cation exchange is the selective complexation of certain metal ions with anionic ligands. This reduces the net positive charge of those ions and decreases their extraction by the resin. In certain instances, where stable anionic complexes form, extraction is suppressed completely. This technique has been utilized in the separation of cobalt and nickel from iron, by masking of the iron as a neutral or anionic complex with citrate350 or tartrate.351 Similarly, a high chloride concentration would complex the cobalt and the iron as anionic complexes and allow nickel, which does not form anionic chloro complexes, to be extracted selectively by a cation-exchange resin. 

Nickel furochlorophin can be demetallated by treatment with concentrated sulfuric acid giving in good yield (60%) the free base the structure of which has also been resolved by X-ray crystallography (Fig. 35c). The macrocycle is more planar than its nickel complex (maximum deviation about 1 A) and can be easily converted into its Cu2 + and Zn2 + derivatives. 

Recent synthetic and spectroscopic work has concentrated on the study of structurally related linear-chain palladium complexes (7, 8), on the possibility of synthesising related nickel complexes, and on the study of mixed-metal mixed-valence complexes of the sort... 

Powdered sodium tetrahydroborate(l -) (0.37 g, 0.01 mol) is added in three portions at about 30-min intervals to a suspension of dichlorobis(tri-isopropylphosphine)nickel (4.5 g, 0.01 mol) in diethyl ether (300 mL) and 95% ethanol (50 mL) in a 500-mL three-necked flask with an argon inlet and a gas bubbler vented to the hood. The reaction mixture is stirred with a magnetic stirring bar at 20° for approximately 5 hr. Gas evolution is observed, and the nickel complex dissolves gradually. After all the nickel compound is dissolved, the solution is filtered into a 500-mL two-necked flask and the solvent is removed under reduced pressure. The yellow residue is extracted with petroleum ether (200 mL), and the solution is washed with two 50-mL portions of water. The petroleum layer is separated, dried with 15 g of anhydrous sodium sulfate, which is washed with two 10-mL portions of the solvent which are added to the petroleum extract the combined extract is then filtered and concentrated in vacuo. The saturated solution (approximately 100 mL) is cooled to —78° in a Dry lce-2-propanol mixture overnight. The crystals which are formed are filtered, washed with a small amount (approximately 10 mL) of petroleum ether at 0°, and dried in vacuo to give brown crystals, mp 65-66° (decomposes). (Yield 2.5 g, 60%.) Anal. Calcd. for Cl8H43ClNiP2 C, 52.1 H, 10.4 Ni, 14.1. Found C, 52.3 H, 9.7 Ni, 14.1. 

Subjecting 2 or 3 to strongly acidic conditions (e.g. 1 M CF3CO2H or concentrated HQ) leads to rapid demetalation and the production of two isomeric free-base macrocycles 10 and 11 [11, 12]. The structure of 10 was conflrmed by its spectral properties and its remetalation to form the nickel complex 5. On the other hand, the structure of 11 was determined solely by its spectral properties. For instance, the NMR spectrum of 11 demonstrated the presence of a fully unsaturated system. Typical also was the presence of pyrrolic proton signals in the 6-7 ppm range of the NMR spectrum [12]. Compound 11 was very unstable... 

The exact nature of the complex from which the final reductive elimination occurs remains subject to speculation. The involvement of a five-coordinate nickel complex RNiLsCN-A appears likely, based on the observation that reductive elimination of benzonitrile from PhNi(Et3P)2(CN) is promoted by adding triethyl phosphite, and that reductive elimination of propionitrile from EtNi[P(0-o-tolyl)3](C2H4)(CN) is first order in triaryl phosphite concentration. ... 

Solubility of Exocellular Nickel Complexes in the Soil. To evaluate the effect of microbial metabolites on Ni solubility in soil, the soluble exocellular solution, separated after growth of the 20 selected bacterial isolates and all fungal isolates, was eluted through a column of soil. The concentrations and forms of Ni in the eluates were compared to the concentrations and forms of Ni in water and in the growth medium before and after microbial growth. A column (0.8 x 4 cm) was packed (1.5 g/cc) with sieved (<2 mm), air-dry Ritzville silt loam (1.0 g) and an aliquot of the filtered (<0.4 M<) exocellular medium (2 ml) was eluted through the soil. After 1 ml of eluant was collected, the Ni concentration was determined from the concentration of radio-tracer. Metal form was characterized in soil eluates by TLC and TLE as previously described. 

Figure 1. Thin-layer chromatographic behavior of nickel complexes with exocellular fungal metabolites (a) before and (b) after elution through soil. Nickel was visualized by autoradiography intensity is directly related to nickel concentration.

Although in most instances CO2 is an inert solvent, it is also the ideal mediiun in which to carry out reactions of CO2 itself. 3-Hexyne in SC CO2 has been shown to react in the presence of Ni(COD)2PPh2(CH2)4PPh2 (COD = cyclooctadiene) to afford the cyclic ester shown (Scheme 1) with the nickel complex acting as a template [51]. The hydrogenation of CO2 catalysed by a ruthenium complex and NEta can be advantageously carried out in SC CO2, where the high concentration of H2 makes enhanced activity possible [52]. 

Nickel may be transported into streams and waterways from the natural weathering of soil as well as from anthropogenic discharges and runoff. This nickel can accumulate in sediment, with the adsorption of the metal to the soil depending on pFI, redox potential, ionic strength of the water, concentration of complex-ing ions, and the metal concentration and type. 

A phosphine-based nickel(II) bromide complex (Ni-2) also induces living radical polymerization of MMA specifically when coupled with a bromide initiator in the presence of Al(0-i-Pr)3 as an additive in toluene at 60 and 80 °C.133 The reaction rates and the effects of radical inhibitors are similar to those with Ni-1, whereas chloride initiators are not effective in reaction control. Additives are not necessary when the polymerization is carried out in the bulk or at high concentrations of monomer, either methacrylate or /v-butyl acrylate (nBA).134 An alkylphosphine complex (Ni-3) is thermally more stable and can be employed for MMA, MA, and nBA in a wide range of temperatures (60—120 °C) without additives.135 A fast polymerization proceeds at 120 °C to reach 90% conversion in 2.5 h. A zerovalent nickel complex (Ni-4) is another class of catalyst for living radical polymerization of MMA in conjunction with a bromide initiator and Al(0-i-Pr)3 to afford polymers with narrow MWDs MJMn = 1.2—1.4) and controlled molecular weights.136 The Ni(0) activity is similar to that of Ni(II) complexes whereas the controllability... 

If, in an early stage of the reaction, a second cyclopropene molecule is coordinated to the nickel, homo-cyclodimerization leading to tricyclic dimers of type 28 may also occur. To prevent the formation of 28, the stationary concentration of the cyclopropene in the reaction mixture must be small, i.e. the cyclopropene must be added slowly. This is especially critical if the electron-poor alkenes are only weakly bound, as is the case with methyl acrylate and the 3-alkyl-substituted acrylates. When acrolein or acrylonitrile are employed, the cycloaddition reaction is inhibited due to the formation of stable bis(alkene)nickel complexes. 

The second example is a hplc experiment in the reversed-phase mode involving the separation of nitrogen bases. Here the stationary phase selectivity is altered for some bases by the addition of a nickel complex to the mobile phase. As demonstrated by LochmUller and Hangac (6) these uncharged, coordinatively unsaturated complexes interact selectively with some bases but not others. Enhanced resolution is acheived at low concentrations (.0001 M) because some bases undergo a 10-fold Increase in retention. The result is... 

In general, the quencher abilities of metal in metal complexes decrease in the order Ni>Co>Cu,Pt. Rates for most of nickel complexes have been observed at near diffusion controlled rate limit and many measurements yield values of the rate constants for total quenching (k + k. ) (23). To determine one of them, the k or k must be measured independently. Recently, we separated k and k by simulation calculation using equations [8] and [9] at various concentrations, as shown in Fig. 7 (29), and examined substituent effect on quenching ability of nickel complexes 1 and 2- The k of neutral nickel complex is nearly... 

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