Aquation reactions

Aquation reactions of some disubstituted aquo ions of Cr(IH) have also been found to be catalysed by Cr viz. 

When the concentration of chloride ion was below 3 M, further aquation reactions from Tc(H20)ClJ to Tc(H20)2C14, etc. were observed. Similarly, aquation of hexabromotechnetate(IV) was studied (molar absorption coefficient, s445 nm = 5720 M "1 cm "1). The equilibrium constants K for Eq. (9) at different temperatures are summarized in Table 2. Analysis of the aquation rate gave the following equation ... 

Figure 7.7 Physiological behavior of cisDDP in vivo. Aquation reactions followed by guanine N7 ligand attachment.

Tab. 4.1 B3LYP/Lanl2DZ-optimized geometric data for all stationary points in the double aquation reaction of cispiatin. The experimental values [56] follow the convention used in the text although no nonbonded ligands are present.

Fig. 4.5 Optimized structures of the first aquation reaction of cispla-tin. The top row shows the reaction starting from a syn arrangement of the nonbonded water molecules.

As mentioned above, results from kinetic experiments [53, 54, 62] are inconclusive about the state of activated cisplatin, and have under some experimental conditions (chloride-depleted environment) revealed the most probable state of activated cisplatin to be the mono-aquated form, cis-Pt[Cl][NH3]2[H20]+. The first aquation reaction has been determined to be approximately two orders of magnitude faster than the second. This opinion is far from undisputed, however, and results from other experiments, based on the ratio of the amounts of DNA adducts formed by the different aquated cisplatin moieties [54], strongly indicates that the likely state of cisplatin binding to DNA is in fact the diaquated state. 

Fig. 4.6 Optimised structures of the second aquation reaction of cisplatin. Both RC2 and RC2a t converge to the same transition struc-... 

Fig. 4.7 Reaction energy surfaces for the first and second aquation reactions of cisplatin starting from (a) the syn arrangement of the initial reactant complex and (b) the...

In a recent publication from our laboratory (87), the substitution behavior of the Cu(II) complex of the trimethylated tren (Me3tren, each amino-terminal nitrogen is monomethylated) was studied. The substitution of the coordinated water molecule by pyridine was only slightly slower than in the tren case. The activation volumes of -8.7 4.7 cm3 mol-1 for the forward reaction and -6.2 1.1 cm3 mol-1 for the reverse aquation reaction (see the volume profile in Fig. 7) indicate that substitution occurs via an associative pathway and that the steric influence is not as significant as in the case of Me6tren. 

Table. Rate Constants for the Thiocyanate Anations of CrLfHjO)" (At,) and the Aquation Reactions (A ) of CrL(NCS)< h-...

EPR, see Electron paramagnetic resonance Equilibrium constants, metal-assisted aquation reactions, 34 168-169... 

When acido groups, halides, thiocyanate, azide, acetate, and nitrate are present in the coordination sphere of cobalt(III), they appear to be oxidized in preference to coordinated ammonia. Many of the radicals thus produced are capable of oxidizing ammonia released from the complex or of interfering in other ways with the reduction process, and these systems have proved very difficult to understand even in general terms. Quantum yield and other data for a number of acidopentammine and certain other complexes are given in Table IV the data on the aquation reactions of some of the complexes are considered in Section I1I-D. 

One important implication of the cage mechanism is that those reactions should be favored for which there is a high probability for the reactants to be in each other s vicinity or cage. Hence, the prevalence of aquation reactions and the importance of ion pairs as intermediates in a nation reactions. I think it is very interesting that John Bailar s examples of stereospecificitv involve rather drastic changes in the immediate environment around the complex since concentrated systems were used in the one set of cases, and dilute ones, in the other. 

Fortunately, there is an experimental test of the validity of this assumption. In the discussion of the aquation reactions, to be presented below, it will be shown that the aquation rate Co(CN)5N3" in alkaline solution is pH independent, clearly indicating that the reverse of Reaction 13 is unimportant. Consequently, the microscopic reversibility restriction requires no appreciable formation of Co(CN)5N3"3 by Reaction 13. 

The aquation reactions of the other complexes do not proceed to completion, even in the absence of added X . Under these conditions two alternative methods may be used to evaluate k. In the first method k is obtained from a study of the initial rate of aquation in the time viterval when the anation reaction may be neglected. In the second method the aquation is studied in alkaline solution where the formation of CoCCN OH-3 tends to drive the reaction to completion. 

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. 

Energy transfer from singlet or triplet states of suitable organic molecules can cause excitation of the central metal ion in a coordination compound. Photosensitization of Cr(III) complexes by biacetyl leads to aquation reaction of Cr(NH3)5 (NCS)2 ion. The aquation of (NH3) is hundred times more than that of (NCS) for the sensitized reaction, whereas it is only 66 times on direct excitation of Cr(IlI). This shows that energy... 

Co(NH8) + is stable in aqueous solution whereas fCo(NH3)5 X)2+ Ihdeigoes moderately rapid substitution of water for the acid group X, f e rates of aquation reaction depends strongly on basicity of X-, for Sample, nitrate acetate is 103 1. The difference in behaviour between Cr(III) and Co(III) ammines may be due to necessity for energy of activation m the transition state for reaction with the latter. 

These aquation reactions follow the same general mechanism as for non-cyclic amines, even though the rates can be many orders of magnitude less.35 The rate expression can show acid independent (solvolytic) and/or acid dependent pathways. For secondary amine/imine macrocycles with less than 16 members, reactions with Ni2+ are usually first order in [H+] (cleavage of first M—N bond rate-determining), while for Cu2+ they are second order in [H+] (cleavage of second... 

Leifer (1988) reviewed fundamental theory and practice of the kinetics of aquatic reactions to express relevant direct and indirect photochemical reactions in natural water. 

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