Slow water exchange complexes

The low-spin t g [Ru(H20)6]3+ is four orders of magnitude more labile than the t g [Ru(H20)6]2+ and exchanges water by an Ia mechanism (125). The slow water exchange on both complexes allowed the direct measurement of the electron exchange of the [Ru(H20)e]3+/2+ couple in acidic solution (Eq. (10)) (133). 

The analysis of 170 NMR and NMRD data on Gd(D03A-monoamide) functionalized dendrimers (generations 5,4 and 3) as well as on Gadomer 17 has shown that, beside fast rotation, slow water exchange also limits proton relaxivity [43,106]. These results show that high molecular weight complexes like these... 

If the water exchange through the membrane is very slow, the main effect will be due to the complex of the outer layer. On the contrary, if this water exchange is extremely fast, complexes in the inner and in the outer layers will equally contribute to the observed paramagnetic relaxation rate as shown in Fig. 25 (115). 

The low-spin t2g [Ru(H20)6] is by four orders of magnitude less labile than the t2g [Ru(H20)6] " (83). An la mechanism was attributed from the negative AV. Because the water exchange on both ruthenium complexes is slow, electron exchange could be measured directly by NMR on the [Ru(H20)6] " couple (27). It has been shown that the self-exchange on the... 

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). 

In another investigation,425 the exchange between [Ce(edta)aq] and hydrated Pb2+, Ni2+ or Co2+ ions again show reaction by dissociation of protonated [Ce(Hedta)aq] as well as by the direct attack of metal ions on [Ce(edta)aq] or [Ce(Hedta)aq]. The kinetic parameters for the Ni2+ or Co2+ ions could be related to the relatively slow (k - 2.6 x 106s 1 for Co2+ and 3.4 x 104 s-1 for Ni2+) water exchange reactions of these ions. The direct attack was interpreted in terms of an intermediate in which one of the carboxylate groups was coordinated to the incoming ion rather than to Ce3+. These reactions were followed by spectrophotometry at 280 nm, where the absorbance of Ce3+aq is much lower than the edta complex. 

DOTA-type complexes exist in two diastereomeric forms (m and M) which may have remarkably different water exchange rates as found for [Eu(D0TAM)(H20)]3+ [58,59]. In this 170 and H NMR study performed in acetonitrile-water solvent, it was possible to detect the NMR signals of the coordinated water molecules in both isomers. In a general case, the observation of the bound water signal for Ln(III) poly(amino carboxylates) is not possible due to the fast exchange, and for Gd(III) complexes, to the slow electronic relaxation. 

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