Rapid water exchange complexes

Enormous acceleration of substitution at ruthenium(II) can be obtained by appropriate choice of ligands. Following the demonstration of remarkably rapid water exchange and complex formation (7d or D in mechanism) at the organometallic aqua-ion [Ru(ri5-C5Me5)(H20)3]2+ (150), comparably rapid substitution has been demonstrated at... 

In the case of Gd3+, there is a rapid water exchange with respect to the relaxation 7im [28]. For water exchange reaction of Gd3+ aquo ion the water tumbling time is 7 x 10 11 s. When Gd3+ is bound to a macromolecule, part of the hydration sphere is substituted by a protein molecule, the effective correlation changes (i.e.) effectively it becomes the electron relaxation time [29] which is about 10-9 s. By virtue of binding to a macromolecule, a net enhancement in proton relaxation rate, Eq is observed which is characteristic of the Gd3+ complex and depends on the resonance frequency and temperature. Some data on the enhancements obtained for Gd3+ protein complexes are given in Table 11.5. 

Complex reducing agents are V(ll), Cr(Il) and Fe(ll). The aqua ions of iron(II) and chromium(Il) with and t2g Cg configurations respectively (high spin d and high spin d ) are kinetically labile undergoing rapid water exchange... 

In relation with the ongoing discussion, if Cu in aqueous solution has five or six water molecules in its first coordination shell (9,116,117), it is interesting to compare water exchange rates measured on five-coordinate copper complexes. Rates of water exchange on five-coordinate complexes of copper(II) are drastically reduced from the rate of exchange on aqua ions of copper(II) (Table VII) (113). The mechanism of water exchange is of associative character in all examples studied to date with the exception of [Cu(tpy)(H20)2]. For that complex the water exchange is very rapid compared to the other complexes and the mechanism is a Id. 

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). 

A number of important structural aspects of zinc complexes as found in enzymes are introduced in this section to serve as background information for the subsequent sections. Aquated Zn(II) ions exist as octahedral [Zn(H20)6] + complexes in aqueous solution. The coordinated water molecules are loosely bound to the Zn + metal center and exchange rapidly with water molecules in the second coordination sphere (see Figure 1) with a rate constant of ca 10 s at 25 °C extrapolated from complex-formation rate constants of Zn + ions with a series of nucleophiles. The mechanism of the water exchange reaction on Zn(II) was studied theoretically, from which it was concluded that the reaction follows a dissociative mechanism as outlined in Figure 2. ... 

---