Nickel complexes water exchange reaction

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). 

The metal ion-water exchange process must be important in areas other than those of simple metal complex formation. For example, the discharge of nickel ion at a mercury cathode is probably controlled, not by diffusion, but by rearrangement of the water coordination shell. The estimated rates and heat of activation for this agree with the idea that this, in turn, is related to the water exchange process (66). Then too, the dimerization rate of metal hydroxy species may be controlled by water exchange. The reaction... 

Dale Margerum Ralph Wilkins has mentioned the interesting effect of terpyridine on the subsequent substitution reaction of the nickel complex. I would like to discuss this point—namely the effect of coordination of other ligands on the rate of substitution of the remaining coordinated water. However, before proceeding we should first focus attention on the main point of this paper-which is that a tremendous amount of kinetic data for the rate of formation of all kinds of metal complexes can be correlated with the rate of water substitution of the simple aquo metal ion. This also means that dissociation rate constants of metal complexes can be predicted from the stability constants of the complexes and the rate constant of water exchange. The data from the paper are so convincing that we can proceed to other points of discussion. 

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. 

In nickel(III) peptide complexes, there is a strong in-plane field provided by the deprotonated peptide linkages (117, 118). Two axially coordinated water molecules are present in the tetragonally distorted complexes which exchange much more rapidly than for the [14]aneN4 species with a substitution rate of constant >106 M l sec-1 for the formation of the imidazole complex (141). However, except for the terminal peptide group, equatorial substitution is very slow. Substitution and rearrangement (125) reactions of these species reveal acid-... 

Reactions in Water.—A preliminary report has indicated that the rate of water exchange in the five-co-ordinate Co complex of (7) is particularly low for this metal (4.2 X 10 s at 25 °C) the activation parameters are given as A/f = 36.5 1.4 kJ mol and AS = — 34 4 JK mol. A preliminary application of conductometric pulse radiolysis to the dissociation of Co -amine chlorides has yielded results consistent with those obtained by other methods. The very small effect exerted by nickel(ii)-bound pyridine on the lability of the remaining water molecules has already been mentioned. An interesting preliminary report has indicated that the substitution reactions of Cu complexes are relatively slow in the absence of Jahn-Teller distortions [as, for example, in the trigonal-bipyramidal complex with (8) see Table 7]. 

Rate results for the two types of ions, cobalt(II) or nickel(II), were normally expected to be similar in a microemulsion solution despite the difference in their reaction rates in aqueous media, because of the reaction limitation by the droplet exchange. This was not observed for the Triton X-100-decanol-water system, suggesting that the droplet collision rate was not limiting the complexation rate and was consequently fast compared to the complexation reaction. 

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