Nickel complexes kinetic studies

Recent kinetic studies of metal ion exchanges include those of europium(iii) with its cydta complex and of various lanthanides(iii) with their respective dtpa complexes. Kinetic studies of metal ion replacement include reactions of nta, edda, heedta, egta, cydta, and dtpa complexes of zinc(ii) with copper(ii), of edta nedta, cydta, and dtpa complexes of lead(ii) with cobalt(ii), of the edta complex of nickel(ii) with indium(iii), and of ttha complexes of cadmium(ii) with lanthanides(in). There are again several systems mainly concerned with 3 + and 4+ ions, for example those involving edta and dtpa complexes of lanthanides. In systems containing ions of high charge, it seems to be easier to demonstrate the existence of dinuclear intermediates of the type invoked in associative (cf. above) pathways. 

The reaction scheme is rather complex also in the case of the oxidation of o-xylene (41a, 87a), of the oxidative dehydrogenation of n-butenes over bismuth-molybdenum catalyst (87b), or of ethylbenzene on aluminum oxide catalysts (87c), in the hydrogenolysis of glucose (87d) over Ni-kieselguhr or of n-butane on a nickel on silica catalyst (87e), and in the hydrogenation of succinimide in isopropyl alcohol on Ni-Al2Oa catalyst (87f) or of acetophenone on Rh-Al203 catalyst (87g). Decomposition of n-and sec-butyl acetates on synthetic zeolites accompanied by the isomerization of the formed butenes has also been the subject of a kinetic study (87h). 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Nickel (II) complexes containing groups of high ligand field strength are often square planar low-spin d8 systems. Many of these are chelate compounds, but several systems of the type [Ni(PRs)2X2] are also known. For the latter complexes no detailed kinetic studies have been reported. The data in Table VIII show that... 

The tetradentate ligand forms monomeric square planar complexes. Synthetic and kinetic studies reveal that the coordinated mercapto group may be converted into the coordinated thioether function without breaking the metal-sulfur bond. The nucleophilic power of the coordinated mercapto group exceeds that of RSH, but depends on the metal atom. Bridging protects the sulfur atom from alkylation. In the case of nickel(ll), alkylation is accompanied by expansion of the coordination number of the nickel from 4 to 6. Ligand reactions have led to the synthesis of planar ligands completely cydized about the metal ion. 

Tetrakis phosphino complexes of nickel(O) readily react with aliphatic and aromatic nitro compounds RN02 to afford the corresponding nitroso complexes of nickel(0) [Ni(PR3)2(RNO)] and the phosphine oxide. Kinetic studies have been carried out to elucidate the mechanism of this oxygen transfer reaction. The reaction mechanism shown in equations (30)-(32) has been postulated.193... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Rate data for the iodination of pyrazole in aqueous solution showed the reaction to be first-order in both iodine and heterocycle and an inverse first-order [H+] dependence was found over the pH range 5.96-6.74 (64JA2857). A kinetic study of the aqueous iodination of pyrazole coordinated to Ni2+ showed the coordinated ligand to react more rapidly, and a [H+] dependence that differed from that of the free ligand (82JA2460). However, the results of this study should be viewed with caution, as the presence of several nickel-pyrazole complexes in solution necessarily leads to uncertainties about the exact nature of the reactive species. 

Although very dramatic rate enhancements have been observed with labile metal ions such as copper(n) and nickel(n), most studies have involved kinetically inert d6 cobalt(m) complexes. In general, copper(n) complexes have been found to be the most effective catalysts for these reactions. 

In the discussion about the structure of selenosalicylideneiminates 496 [884], it was emphasized that the corresponding nickel complex is cis-planar, while the related cobalt and zinc compounds have a tetrahedral structure. Kinetics of enantioisomery in chelates 496 (M = Zn, Cd, Pb R — <-Pr) have been studied in detail [884b],... 

Recent kinetics studies on protonation of [Ni(SEt)((Ph2 PCH2CH2)2PPh)]+ (14) proposed that the proton interacts with both the nickel and sulfur sites,consistent with the proposal of intramolecular proton transfer between cysteinate sulfur and Ni atom in the Ni-based hydrogenases. Additionally, the mononuclear complex [Ni(psnet)]+ (15) of known structure and a mildly negative redox potential can stoichiometrically evolve H2 from protic sources. On the basis of kinetics analysis, the reaction paths considered most probable involve steps of protic oxidative addition to Ni(I) to generate Ni -H , and electron transfer to Ni(III) followed... 

Careful and extensive kinetic studies have been carried out only for hydroaluminations with dialkyl-aluminum hydrides. Adequate kinetic information is still lacking on transition metal catalyzed hydroalumination and on the hydroaluminating action of complex metal hydrides, MAlRnH4 n. Preliminary studies on the nickel-catalyzed process have revealed an unstable rate behavior brought about by the deactivation of the catalyst with time. ... 

Shippey and Donahue [11] were the first to show how to derive an empirical expression for the overall rate law for electroless deposition reactions. They studied an electroless copper system with tartrate as a complexing agent. Later, Molenaar et al. [12] performed similar kinetic studies concerning an electroless copper deposition reaction with EDTA as a complexing agent. The kinetics of electroless nickel deposition was investigated by Mallory and Lloyd [13]. 

A kinetic study of the hydrolysis of JV-salicylideneaniline (113) in the presence and absence of cobalt(II), nickel(II), copper(II) and zinc(II), using 10% ethanol-water as solvent, has been carried out by Dash and Nanda. The (1 1) Schiff base-metal complexes (ML ) were found to undergo acid-catalyzed hydrolysis at rates decreasing with the thermodynamic stabilities of the complexes, the most thermodynamically stable complexes undergoing the slowest rate of hydrolysis. More recent measurements using high copper(II) to ligand ratios have indicated that the copper(II)-imine is quite stable to hydrolysis at pH 5. 

Several kinetic studies have been reported on nickel(II) complexes including acetonitrile exchange (134, 136) with complexes formed by nickel(II) and 2,2, 2"-triaminotriethylamine, (130) 2,2, 2"-tri(V V-dimethylamino)-triethylamine, (130) 2,2-di(hydroxymethyl)-1 -propanol, (135) 2,2-di(aminomethyl)-l-propylamine, (135) 2,2-di(aminomethyl)-1-propanol (135) and 2,2, 2"-trihydroxytriethylamine. (135)... 

Mixed Donors. The 4-methylpyridine adduct of bis-[l-(2-thienyl)-4,4,4-trifluoro-butane-l,3-dionato]nickel(ii) is shown to have weaker co-ordination than the nonadduct by X-ray studies. The crystal structure of bis-(8-amino-2-methylquinoline) nitratonickel(ii) nitrate shows that the nitrato ligand is bidentate (66). A kinetic study has been made of nickel(ii) murexide complex formation in DMS0-MeN02- Nickel(ii) complexes of some optically active ethylenediamine-NN -diacetic acid-type polyaminocarboxylic acids have been prepared and solid-state spectra and t.g.a. recorded. N.m.r. temperature dependence for racemization of Ni(edta) ,... 

In a kinetic study, the reaction was found to be first order in MVN and HCN over concentration ranges below 0.04 M in each reagent. This saturation kinetics im-pHes that the catalyst resting state shifts from Ni-[la]-(COD), 5 (Scheme 5), to either 8 or 9. Based on the known stabihty of the 18-electron allylic hydrocyanation intermediates (vide supra) and the exclusive regioselectivity of this reaction, we beheve that complex 9 is the catalyst resting state under most hydrocyanation conditions. Under these saturation conditions, a maximum activity of 2000 tuxnovers/h (turnover=mol of nitrile/mol of nickel) was observed for the Hgand la. One of the minor comphcations of the reaction is the catalyst deactivation which removes Ni(0) from the system by an oxidative addition of HCN to form Ni(CN)2. A practical consequence of this side reaction is that the catalyst life time is reduced to 700-800 turnovers, unless a fresh supply of Ni(COD)2 is introduced into the medium. 

By the late 1950s, Chatt and Bernard Shaw had succeeded in preparing several alkyl-, aryl- and hydrido-metal complexes of the nickel triad. Because of our mutual interest in the kinetic trans effect of platinum(ii) complexes, Chatt and I decided to examine the rates of ligand substitution of these new organometallics. Shaw prepared the compounds and Harry Gray did the kinetic studies. Years later Chatt gave the following account of why the research was so rapidly accomplished. 

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