Nickel coordination sphere

In summary, prominent features of ylide nickel complexes versus phosphane complexes have been identified an electron-rich nickel center, energetically destabilized nickel-localized occupied orbitals, a significant weakening of the Ni-O bond, the phosphoms moiety being located outside the nickel coordination plane, thus opening one axial position in the nickel coordination sphere for easy monomer landing . 

The catalyst plays the most important role in the SM reaction as the reaction actually proceeds within the palladium or nickel coordinative sphere. Since only Pd(0) and Ni(0) complexes possess the catalytic activity, they can be used by ... 

Results from an array of methods, including X-ray absorption, EXAFS, esr and magnetic circular dichroism, suggest that in all ureases the active sites are a pair of Ni" atoms. In at least one urease,these are 350 pm apart and are bridged by a carboxylate group. One nickel is attached to 2 N atoms with a fourth site probably used for binding to urea. The second nickel has a trigonal bipyramidal coordination sphere. 

In the calculations of the energy of hydration of metal complexes in the inner coordination sphere, one must consider hydrogen bond formation between the first-shell water molecules and those in bulk water, which leads to chains of hydrogen-bonded water molecules. Such hydrogen-bonded chains of ethanol molecules attached to the central metal ion have been found as a result of DFT B3LYP calculations on ethanol adducts to nickel acetylacetonate, where the calculated energy of hydrogen bonds correlated well with experimental data [90]. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

If the electronic spin state change were the critical determinant of the dynamics of spin equilibria, then the AS = 1 equilibration between planar and tetrahedral nickel(II) isomers would occur more rapidly than equilibration of the octahedral AS = 2 spin states. This is not observed. Even though the AS = 1 transition is most likely adiabatic, the large coordination sphere reorganization energy requirement causes these nickel(II) isomerizations to occur relatively slowly, with relaxation times of the order of a microsecond. 

The most important azo compounds employed in the manufacture of dyes of this type are those containing the < ,o -dihydroxyazo-, the o-hydroxy-o -carboxyazo- and the o-hydroxy-o -amino-diarylazo systems. It is well established3 33-0 that these form four-coordinate copper and nickel complexes (35) in which the coordination sphere of the metal can be completed by a variety of neutral ligands. In both cases the light-fastness of the parent azo compound is improved as a result of complex formation but the nickel complexes are insufficiently stable towards acid to be of commercial interest as dyestuffs. The history of copper complexes has already been discussed (Section 58.1) and will not be considered further here, although it is worthy of mention that currently the most important copper complex dyestuffs are those containing fibre-reactive systems, e.g. (36), for application on cellulosic fibres. 

In recent years there has been considerable interest in the reactions of nitriles in the coordination sphere of metal ions. Breslow et al.312 first reported that the hydrolysis of 2-cyano-l,10-phenanthro-line to the corresponding carboxamide is strongly promoted by metal ions such as copper(II), nickel(II) and zinc(II). Base hydrolysis of the 1 1 nickel complex is 107 times faster than that of the uncomplexed substrate. The entire rate acceleration arises from a more positive value of AS. Somewhat similar effects have been observed for base hydrolysis of 2-cyanopyridine to the corresponding carboxamide. In this case rate accelerations of 109 occurred with the nickel(II) complex.313... 

Considering the reductive elimination mechanism that takes place within the coordination sphere of the palladium, one might expect the nucleophilic addition of unstabilized nucleophiles to be more enantioselective than that of stabilized nucleophiles because the nucleophile can directly interact with the chiral ligand. However, there are only a few examples in the literature that give high enantioselectivity. In the case of the alkylation with unstabilized carbanions, nickel catalysts have been more frequently used (see next section). 

Insertion of new ligands into metallocomplex systems may proceed reversibly. Being reduced in the framework of the complex, these ligands lose the ability to be coordinated and leave the coordination sphere as products. One important example of such ligand sliding is the catalytic transformation of C02 into CO. Rhenium, palladium, platinum, and nickel complexes were recommended to catalyze this process (Hawecker et al. 1986 Du Bois Meidaner 1987). The Ni(II) complex with 1,4,8,11-tetraazacyclotetradecane is preferential (Beley and co-authors 1984). 

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