Chloride aquation rate

Influence of Amine Nonleaving Groups on Chloride Aquation Rate Constants ... 

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 ... 

It is easy to argue that the behavior of the [Co en2 Cl2]+ isomers is not surprising. The correlation of aquation rates of complexes of the type, [Co en2 A Cl]n+ with the electron displacement properties of the nonparticipating ligand, A, has led to the belief that ligands able to donate a second pair of electrons to the metal can thereby stabilize the 5-coordinate intermediate and hence promote a unimolecular reaction (2, 18, 24). Chlorine is such a ligand, Cl—Co- -Cl, and the essentially first-order kinetic form could be used as evidence for a unimolecular mechanism, once the ion association pre-equilibrium effects for the displacement of chloride under the electron-displacing influence of the other chlorine atom have been taken into account. 

In all of the systems so far investigated the aquation rate has been found to be pH independent in alkaline solution, at least up to 0.1MOH ", the largest concentration investigated. It may be noted that this behavior is entirely analogous to that of trityl chloride and other organic halides which undergo solvolysis by well established Svl mechsnisms. (S)... 

These range from [RuQJ3- to [RuQ(H20)5]2+. The aquation rates from the hexa-chloride vary enormously 13... 

Water Exchange, Chloride Aquation and Chloride Anation Rates for [MlNHalsX)" Compounds ... 

The synthesis and reactivities of substituted aliphatic bidentate complexes are similar to the reactions of analogous ethylenediamine complexes. Basolo and co-workers prepared and studied the rates of chloride aquation of a variety of fra i-[Rh(NN)2Cl2] complexes (NN = /neso-bn, where bn = 2,3-diaminobutane (+)-bn, and tetrameen—both cis and trans isomers) the rate of chloride release is not dramatically dependent on the nature of the aliphatic amine ligand. 

As in organic chemistry where m values for bromides are rather lower than for chlorides, for example m — 0.92 for t-butyl bromide compared with m = 1.00 for t-butyl chloride, m values for cobalt(ra)-amine-bromide complexes are, at around 0.2, rather lower than for the analogous chlorides. Whereas in this work the effect of solvent structure on reaction rates has been used to gain further insight into reaction mechanisms, the opposite approach has also been used, in a study of aquation of tra/u-[Co(en)2Cl2]+ in alcohol-water mixtures, in which variation of rate with solvent composition has been used as a probe of solvent structure variation. Rates of aquation of both cis- and trans-[Co ea)2C have been determined in aqueous acetonitrile (0 < mole fraction MeCN < 0,104). For both complexes aquation rates decrease only slightly as the proportion of acetonitrile increases, with the cir-complex slightly more sensitive to solvent variation. The kinetic effects observed here are smaller than those observed in t-butyl alcohol-water solvent mixtures. ... 

Aquation rates increase in the order chloride < bromide < iodide, which is the opposite order of reactivity to that observed for the platinum(u) complexes [Pt(NH8)2Xa]. The order of reactivity of the platinum(iv) complexes reflects a connection between lability and reducing power of the halide ligand. A polarographic study of the reaction of [PtClgCOHa)]" with nioxime indicates rate-determining loss of water followed by reduction to platinum(n) the product is [Pt(nioxH)2Cl2]. The activation parameters are = 19.3 kcal mol and AS = —19.2 cal mol deg. Irradiation of aqueous solutions of [Pt(NHs)4(NH2)(N02)] + results in photoisomerization to the nitrito-complex and redox reactions, as well as photoaquation with replacement of the nitro-ligand. Quantum yields are reported, but no kinetic results. ... 

A preparative study of the cis and trans isomers of [Ru(LL)2Cl2], where LL = diars or diphos, indicates that the cis isomers are, as usual, considerably more labile than the trans isomers. The cis isomers readily undergo sterespecific substitution by iodide or by carbon monoxide. Some qualitative observations on lability of chloride coordinated to ruthenium(II) is also available for several /i-chlorodiruthenium species. The Ru(II)Ru(II) form (n = 4) of the /i-dinitrogen series [(H3N)5RuN2Ru(NH3)5]" is very inert the Ru(II)Ru(III) form (n = 5) has an aquation rate constant of 0.024 or 0.1 s , while the Ru(III)Ru(III) form (n = 6) has an aquation rate constant s 10 s . ... 

Co(NH3)5C1] + has been the subject of controversy for some years. The effects of added chloride on aquation rates of this complex were first studied up to a concentration of 0.1 M sodium chloride, later up to 0.6M sodium chloride. The results of the latter study, in which sodium perchlorate was used to maintain constant ionic strength, were subsequently reinterpreted in terms of three parallel /d reaction pathways, involving chloride and perchlorate ion pairs. Now the effect of added chloride on the aquation rate of the [Co(NH3)5C1] + cation has been investigated at chloride-ion concentrations up to 0.9M. The conclusion reached from this latest and most extensive study is that neither [Co(NH3)5Cl] + Cl nor [Co(NH3)5Cl] + ClOr ion pairs play a kinetically significant role in the aquation of the [Co(NH3)5Cl] + cation under experimental conditions so far used. Moreover the aquation rate constant for this complex is unaffected by the nature of the added anion, be it perchlorate, tetrafluoroborate, or trifluoromethylsulphonate, when the respective sodium salts are used to maintain the ionic strength of the reaction medium (/ = l.OM). ... 

The above results show that the reactions of all organocobalt(III) complexes with Hg(II) ions so far reported share several features in common. The reaction proceeds by an Se2 mechanism and the rate is reduced [compared to that of the simple aquated Hg(II) ion with the methyl complex] by (1) complexing of the Hg(II), e.g., with chloride, (2) increased substitution on the a-carbon, and (3) reduced electron donation from the cis and/or trans ligands. 

Figure 4 Hydrolysis scheme for cisplatin-based anticancer agents. Where L = U = NH3 cisplatin is indicated. When L = NH3 and V = 2-picoline or cha the rates of aquation of the trans-chloride ligands are different.

In aquatic environments, Spear (1981) spotlights three research needs (1) development of analytical procedures for measurement of individual dissolved zinc species, notably the aquo ion and zinc chloride, and for nondissolved species that occur in natural waters (2) separation of natural from anthropogenic influences of sediment-water interactions on flux rates, with emphasis on anoxic conditions, the role of microorganisms, and the stability of organozinc complexes and (3) establishment of toxicity thresholds for aquatic organisms based on bioaccumulation and survival to determine the critical dose and the critical dose rate, with emphasis on aquatic communities inhabiting locales where zinc is deposited in sediments. These research needs are still valid. 

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