Nickel ligand exchange

Nickel. Ligand exchange in [Ni2(dppm)2(C=CH2)X X ] has been studied and rate constants determined using P EXSY NMR spectroscopy. 43 interchange of isomers of [Ni(OR)2(PMe3)2] has been investigated. 44... 

The larger bite angles of the C3-bridged ligands preclude the ligand-exchange equilibrium, the ionic radius of the nickel(II) ion being too small to accommo-... 

Scheme 5.4 Three processes in which nickel diphosphane complexes can be involved under catalytic hydrogenation conditions. (A) ligand exchange (B) oxidation and (C) hydrogenation of 1-octene. For simplicity, the reac-...

Nickel(II) complexes with /3-ketoamines are, in general, easily prepared. The most useful and general synthetic methods are the following (i) reaction of the preformed ligands with nickel salts in basic solution using water, alcohol or their mixtures as medium (ii) ligand exchange reactions (iii) template reactions. Complexes of type (329) may be sensitive to moisture and are prepared in anhydrous conditions. 

Much more research has been carried out with polymers in which the coordinated metal atom is part of the chain backbone. Typically, the metal atoms are copper, nickel, and cobalt. Oxygen atoms or carbon atoms adjacent to the metal atom provide the electrons required for the coordinate bond.30 Polymers of this type are often rather intractable, for a variety of reasons. Specifically, insolubility can be a problem for species with moderate molecular weights. Also, coordination between chains can cause aggregation, and ligand-exchange reactions with small molecules such as solvents can cause chain scission. However, in some favorable cases, the intramolecular coordination is sufficiently strong for the polymer to be processed by the usual techniques such as spinning into fibers or extrusion into films.30... 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

Similar mechanisms may be proposed for the hydrolysis of amino acid esters and amides co-ordinated to labile metal centres such as copper(n) or nickel(n), although mechanistic studies at these centres are much more difficult to perform in view of the rapidity of ligand exchange processes. Further complications arise from the formation of insoluble or colloidal suspensions of metal oxides and hydroxides at higher pH values. In general,... 

At the present time the preparation of the trifluoromethylated derivatives of low valent transition metals by ligand-exchange reactions appears to be quite general. However, as exemplified by the nickel reaction above, the utility of the method is obviously subject to the inherent stability of the desired product. In many cases, such as the preparation of the trifluoromethyl derivatives of the cyclopentadienyl cobalt system, (CF3)Co(Cp)(CO)I and (CF3)2Co(Cp)(CO), the reaction of the dihalide with (CF3)2Cd glyme represents the simplest reaction... 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

The preparations described here are developed from published work by Malatesta et al.5 and from more recent studies in the contributors own laboratory.2 The cobalt and nickel complexes are prepared by reduction of the corresponding metal nitrates with sodium tetrahydroborate in the presence of excess ligand, whereas the syntheses of the rhodium and platinum complexes involve simple ligand exchange processes. The preparative routes are suitable for use with triphenyl- or p-substituted triphenyl phosphites reactions involving o- or m-substituted triphenyl phosphites give much reduced yields of products which are difficult to crystallize and are very air-sensitive. These features probably reflect the unfavorable stereochemistry of the o- and m-substituted ligands. 

To obtain the HMC as an active component zero-valent nickel complexes of the general formula - Ni[PRj] (n=2-4), where R was Ph or EtjN, characterized by high activity in oligomerization of lower olefins in homogeneous conditions were taken. Heterogenization of these complexes was conducted by method of ligand exchange. In the Literature there are examples of carbonyl complexes of palladium prepared by this method, which show activity in propylene dimerization [8], However, we have failed to find data on such nickel catalysts in the literature. 

The complex formation between Ni(ii) and acetate ions in aqueous solution has been studied by C NMR. Equilibrium quotients (132) and rate parameters (133) for the ligand exchange processes are reported. Complexes of hydroxy-acids (134) and iminodiacetates RN(CH2C0 )2 (135) have been the subjects of further studies. Co(ii) complexes of malate, citrate, isocitrate, and monomethylcitrate all exhibit large shifts which may be accounted for by assuming a common structural unit [22] in which the ligand is tridentate. Nickel(ii)... 

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