Aqueous nickel halide complexes

Besides the weakness of complex formation, the simultaneous presence of inner- and outer-sphere complexes, especially in the cases of chloride and bromide, further complicates the determination of the correct solution speciation. Using most of the experimental methods the measured formation constant is the sum of the formation constants of two types of complexes, [NiL(H,Q),l and [Ni(H20) L] ,  

However, the d-d transitions of Ni(II) are certainly more influenced by inner-than outer-sphere complexation. In the case of weak complexes, inner-sphere interaction can be expected to dominate only at markedly reduced water activity, i.e., in case of concentrated solutions. Consequently, a visible spectrophotometric study, using high concentrations of the complex-forming anion, mostly provides information on inner-sphere complexes, and it is therefore not strictly comparable with e.g., potentiometric results obtained at considerably lower ligand concentrations. 

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It is well-established that electroreduced nickel(I) complexes of cyclam and a variety of substituted cyclams add oxidatively to alkyl halides to give alkylnickel(III) complexes in organic solvents,251,276 the lifetime of the carbon-nickel bond governing the overall behavior of the system. However, it was shown that [Ni (tmc)]+ (one-electron reduced form of complex (17) tmc= 1,4,8,11-teramethyl 1,4,8,11-tetraazacyclotetradecane) reacts with alkyl chlorides in aqueous alkaline solution in a one-electron process.277,278... 

AHR/ROS] Ahrland, S., Rosengren, K., The stability of metal halide complexes in aqueous solution. II. The fluoride complexes of divalent nickel, copper and zinc, Acta Chem. Scand., 10, (1956), 727-734. Cited on pages 142, 143, 283. 

Of particular interest are the solubilities of cobalt-, nickel- and copper-halide complexes in dense supercritical aqueous phases. Aquo- and halide complexes have higher coordination numbers at higher density. Several of gaseous solutions are highly coloured (for example dark blue cobalt-II solutions). Visible and UV-spectra, which permit to determine the stability ranges of various complexes have been reported by us up to 500 °C and 6000 bar [43-45]. The results may have interest for hiture catalyst developments. We have used near-UV spectroscopy to determine the solubility of organic solids in various dense gases. Examples are anthracene [46] and caffeine [47] to 200 °C and 2000 bar. Supercritical extraction has become a widely applied procedure in the food, pharmaceuticals and other industries. 

The transition from localized behavior to delocalized behavior is an important one. As an example, nickel oxide (NiO) has about the same color as Ni + complexes in aqueous solution. Cu + complexes in aqueous solution are blue, but teud to be green if the ligands are halide ions or violet if the ligands are ammonia molecules. Copper(II) oxide (CuO), on the other hand, is black. Very likely, the wide absorption in the visible region is from an allowed transition that covers the weak ligand field transitions. It cannot be an LM transition since there is no reason to believe that LM transition are in the visible region just in the case of CuO. 

In aqueous solution, the complexes of most metal cations exist in dynamic equilibrium with their components. If we disturb this equilibrium, another one is instantly formed. It is quite otherwise with robust complexes which persist for hours (or even days) under conditions favourable to their decomposition any biological properties that they may have are strikingly different from those of their components. Robust complexes are formed where metal ions have 3,4 (low spin), 5, or 6 d electrons provided that formation of the complex involves large values of ligand-field stabilization energy. Metals most prone to form robust complexes are the transition metals platinum, iridium, osmium, palladium, rhodium, ruthenium, also (but not so frequently) nickel, cobalt, and iron. The halide and, particularly, the cyanide anions most readily form robust complexes with these transi-... 

An important alternative to the Ullmann method is the use of certain nickel complexes,193 This method has also been used intramolecularly.194 Aryl halides ArX can also be converted to Ar—Ar195 by treatment with activated Ni metal,196 with Zn and nickel complexes,197 with aqueous alkaline sodium formate, Pd-C, and a phase transfer catalyst,l9li and in an electrochemical process catalyzed by a nickel complex,199... 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

In general for carbonylations, palladium as catalyst metal is preferable to nickel with respect of catalyst efficiency. Thus, Okano, Kiji, and co-workers described some other efficient palladium-catalyzed carbonylations of allyl chloride and substituted allyl halides (Eqs. 5-10). In greater detail, the water-soluble palladium complex PdCl2[Ph2P(w-C6H4S03Na)]2 has been used in a two-phase system (e.g., aqueous NaOH/benzene medium) at atmospheric carbon monoxide pressure, giving 3-butenoic acids [20], In the carbonylation of allyl chloride a mixture of 2-bute-noic acid, which was formed by base-catalyzed isomerization, and 3-butenoic acid was obtained in up to 90% yield (TON = 135), albeit at moderate selectivity (24 76). Clearly, the isomerization depends on the concentration of the base and was therefore suppressed by a method of continuous addition to the aqueous medium. 

In this section, we discuss atomistic modeling studies of the interaction of model passivated surfaces with halide ions. First, the study using conventional DFT of chloride adsorption and subsurface penetration on defect-free hydroxylated nickel oxide surfaces characteristic of passivated nickel surfaces is presented. Then the implications of using DFT-l-U are discussed as well as the interaction with other halides. This is followed hy one example on the effect of implementing surface defects characteristic of those observed experimentally on a passivated nickel surface. Finally, the application of reactive MD modeling to more complex systems including a substrate metal (copper) covered by a passive film (copper oxide) in interaction with a chloride-containing aqueous solution is presented. 

In a study of racemisation of tris(bipy) and tris(phen) complexes of iron(II) and nickel(II) in the solid state, further evidence was obtained to support the assignment of an associative-type mechanism, with a transition state or intermediate of coordination number 7, for racemization at nickel(II), but of racem-ization via a trigonal prism intermediate for the iron(II) complexesA useful freeze-drying technique has been described for the isolation of optically active solids in preparations of halides of cations of this type/ Different kinetic patterns, and therefore the likelihood of different mechanisms, have been established for the racemization of [Fe(phen)3] and [Ni(phen)3] in water containing small amounts of nitrobenzene. The extent of association between the cations and the nitrobenzene was estimated from determination of the solubilities of nitrobenzene in aqueous solutions of [M(phen)3]Br2. Complementary determinations of the solubilities of [M(phen)3]Br2 in aqueous nitrobenzene might have permitted an initial-state-transition-state dissection to have been carried out. 

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