Exchange at Nickel

The planar complex reacts much faster than the other two, with a negative AS suggesting associative activation. The substitution of R-sal by H2-salen for the other two complexes appears to be dominated by rapid planar/octahedral equilibria in solution, and the octahedral form is kinetically inert. The rate differences span four orders of magnitude in acetone, but are reduced by a factor of 30 when MeOH is employed as solvent. This is probably caused by coordination of the more polar methanol, leading to more stable octahedral adducts. Interestingly, the observed rate constant for the reaction of the planar complex is greater in methanol than it is in acetone. The likely explanation is a contribution from an associative solvolysis pathway in the more polar solvent. 

The nickel(II) macrocyclic ligand complex (4) is diamagnetic and planar as the perchlorate, but paramagnetic and octahedral with additional ligands such as 

2(NCS ), 2(N3), acac , or en. In dmso or dmf, the planar form slowly converts to an equilibrium mixture dominated by octahedral bis-solvates. Activation enthalpies are about lOOkJmor The slow rates and high A// indicate reactions involving Ni—N bond breaking/  

---

Figure 4 is probably the most appropriate form for associative ligand exchange at nickel(II) complexes, well represented by 5-coordinate structures of both types 54, 55). 

Other Solvents. Two recent determinations of rates and activation parameters for dimethyl sulphoxide exchange at nickel(ii) give tolerably consistent results, but there is a disturbing lack of agreement between these... 

It is only rarely possible to probe the composition or structure of secondary solvation shells. One situation where this is possible is [Cr(NCS)6] in solution in aqueous acetonitrile, described in Section 3 of Chapter 4. Control over solvent shell composition in mixed solvents is most readily achieved by the use of mixtures of co-ordinating and non-co-ordinating solvents. Examples of applications of this include the study of DMSO exchange at nickel(ii) in DMSO-nitromethane and DMSO-methylene chloride mixtures, and of aquo-ions in methylene chloride. ... 

Kinetics of ligand substitution and solvent exchange at nickel (ll) ion in various solvents... 

Some time ago it was reported that rates of dimethyl sulphoxide exchange with nickel(ii) in dimethyl sulphoxide-dichloromethane and dimethyl sulphoxide-nitromethane solvent mixtures were almost independent of solvent composition. Recently, rates of dimethylformamide exchange at nickel(n) in dimethylformamide-nitromethane solvent mixtures have also been found to be remarkably insensitive to solvent-composition variation. Whereas in the dimethyl sulphoxide-dichloromethane solvent mixtures nickel(n) exists as [Ni(DMSO)e] +, in dimethylformamide-nitromethane mixtures the composition of the nickel(n) co-ordination shell varies with solvent composition, with preferential solvation by the nitromethane rather than by the dimethylformamide. These observations are difficult to explain in terms of an /d... 

Fig. 19. Rate of H2-D2 exchange (at -40°C) divided by nickel content as a function of composition for Cu-Ni alloy films deposited at 300°C and sintered at 400°C 84).

A mechanism which proceeds through surface reconstruction of the substrate has been identified for Ni deposition on Au(lll) [120, 121]. The process begins with place exchange of nickel into a particular position in the reconstructed Au(lll) surface, followed by deposition of Ni islands on top of the imbedded atom. At higher overpotentials, nucleation occurs instead at step edges, so that control of the potential allows control of the nucleation process and the distribution of Ni in the early stages of growth. In this instance, the nucleation process has been captured by STM on the atomic scale. 

Trimetrexate (88) Is an antineoplastic agent related to the well-established folic acid antimeta-bolite methotrexate. It can be synthesized by selective diazotization of the most basic amino group of 2,4,6-triamino-5-methylquinazoline (85) followed by a Sandmeyer displacement with CuCN to give nitrile 86. Careful reduction asing Raney nickel produces the aminomethyl intermediate 87 or, if the reaction is carried out in the presence of 3,4,5-triniethoxyaniline, trimetrexate (88) [24]. One presumes that that outcome is a consequence of amine exchange at the partially reduced imine stage and further reduction. 

Hydrogen is chemisorbed on a surface partially covered with ethylene and reacts rapidly with the ethylene at room temperature. This is not consistent with observations that hydrogen-deuterium exchange on nickel is strongly inhibited by the presence of chemisorbed ethylene. The latter effect may be due to fragmentation of ethylene which occurs only at higher temperatures. 

Adsorption geometry could cause the observed rate difference. Crawford and Kemball have found that deuterium exchange of the two methyl groups of the isopropyl side-chain in cumene over nickel films occurs in two steps (32). Both methyl groups cannot exchange at the same time. The cumene molecule probably must leave the surface and re-adsorb before the second... 

For the Ru system the thiol hydride could not be detected, while for the Rh system and also [IrH2(HS(CH2)3SH)(PCy3)2]+(which similarly catalyzes D2/H + exchange (79)), the H2 complex could not be seen but is a transient. A related system, Ni(NHP Pr3)(S3) clearly shows that heterolysis of D2 can also occur at nickel sites, which may be relevant to H2 activation in [FeNi] hydrogenases (78). 

The catalysts studied were prepared and purified by methods previously described1. Briefly, the silica-alumina support was prepared by a coprecipitation method on to the support and possessed a Si/Al ratio of 25. Ni2+ ions were introduced from an aqueous solution of nickel chloride by ion exchange at reflux. The final catalysts contained about 1.5 mass% Ni, with some residual sodium and chlorine, corresponding to a CI Ni mol ratio of <0.1, and a Ni Al mol ratio close to 1.0 (calculated with the assumption that all residual sodium is associated with some of the Al sites in the material (Na/Al mol ratio typically ca. 0.5)). 

Catalyst Preparation. Catalyst preparation consisted of the exchange of nickel or cobalt nitrate for the sodium cation. Ratios of nickel or cobalt to sodium of 20 1 were used for maximum exchange and the ion exchanged zeolite pellets were leached with deionized water. Catalyst preparations were reduced for 16 hours at 400°C in a stream of hydrogen. Similar procedures have been reported (5, 6). 

Only for nickel(II) does the prospect of prereaction geometry change pose a potential complication and since more examples of faster associative nucleophilic ligand exchange at square-planar complexes (rather than their tetrahedral, 5-coordinate, or octahedral alterna-... 

Fig. 23. Catalytic activity of aluminosilicate catalyst exchanged with nickel ion for disproportionation and hydrodeinethylation of toluene versus amount of nickel in the catalyst. Reaction pressure, 1 atm reaction temperature, 550° LHSV, 2 hr 5 molar ratio of H2 to feed percent conversion, values at 8 ml (A) and at 40 ml (B) of total feed. All catalysts were calcined at 550° in the air before use.

In the second series of experiments, the exchange of oxygen between adsorbed carbon monoxide and the lattice anions was followed at room temperature, either carbon monoxide (64) or nickel oxide (65) being labeled by 0. When the pressure of carbon monoxide is 80 torr, 20% of surface anions are exchanged at room temperature (64). The extent and the rate of this reaction are not modified if oxygen is preadsorbed on the sample before the exchange experiment. It has already been shown from the infrared absorption spectra (60) that no carbon monoxide is adsorbed irreversibly as CO on the surface cations of an... 

---