Aquation rate constants

Table 2 [Co(NH3)5X] 3 -,+ aquation rate constants for anionic and neutral leaving groups (Xn, n = 0-3, at...

The aquation rate is mainly determined by the trans effect of the ligand trans to Cl. The aquation rate constants of some platinum complexes are listed in Table I. For example, the rate of hydrolysis of CP trans to c-C6HnNH2 in complex 14, a metabolite of the oral drug 11 (42), is about twice as fast as trans to NH3 (43). 

Aquation rate constants [Co(NH,), X]", 34 165-166 ion pairing effects, 34 173 metal-assisted reactions, 34 168-169 pentaamminecobalt(III) complexes, 34 149 pentaamminemetal(III) complexes, 34 152-153... 

As for the thermal isomerization of cw-Cr(en)2(OH)2+, our values of k are about twice those of Olson and Garner (although for somewhat different substrates), and our activation energy of 35 kcal. is definitely higher than their 32 kcal. However, using parallel runs with cis and trans isomers allowed us to correct for aquation, and made the values of K and k less interdependent. As noted under Experimental, the latitude in K values, stemming from the presence of some aquation (K and the aquation rate constant are again interrelated), was considerable. We can only delimit AH° of isomerization as 0 =fc 2 kcal. 

Scheme 2. Mechanism of Formation of Cisplatin-DNA Adducts. kx, k2, k3 are aquation-rate constants (fcj and k2 are in Scheme l).kpl and kp2 are platination-rate constants, kc is the chelation-rate constant.

Table 7. Chelation-Rate Constants of Aqua-Monoadducts and Aquation-Rate Constants of Chloro Monoadducts (ci.v-oligoG-Pt(NH3)2Y, Y=H20, Cl) of Various Oligonucleotides...

Aquation Rate Constants for (Ligand)Pentaamminecobalt(III) Complexes... 

The [M(NH3)5(0S02CFa)] + ions actually show an increase in relative lability, compared with [M(NH3)5C1]2+ ions, for second- and third-row members of the cobalt triad the ratio of aquation rate constants (CF3S03")/ (C1 ) varies from 1.4 x 10 (Co) to 5.0 x 10 (Rh) and 2.1 X 10 (Ir) (84). Likewise, the ratio of aquation rate constants for triflate and nitrate is substantial, with (CF3S03 )/ (N03 ) varying... 

Aquation Rate Constants for Selected (Ligand)Pentaamminemetal(III) Complexes"... 

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

Spectacular increases in spontaneous and base-catalyzed or metal-assisted aquation rate constants of particularly pentaammineco-balt(III) complexes have been observed in the presence of polyelectrolytes and micelles. Polyelectrolytes are known to cause marked acceleration of reactions, though decelerations can be observed, and the effects are very large compared with those produced by equivalent amounts of corresponding low molecular weight substances similar effects are observed in solutions containing charged micelles. This area of research has been extensively reviewed (63,101,133,139,195, 206, 207), so discussion will be selective. 

Since substitution reactions of square-planar complexes in most cases tend to follow an associative mechanism, a large number of complexes were synthesized in efforts to determine whether a changeover in mechanism is induced. The increase in steric hindrance caused by introducing alkyl substituents on the three N donor atoms of diethylenetriamine (dien) in going from dien to Mesdien to Etsdien, caused a decrease in the aquation rate constant for [Pd(Rsdien)Cl]+ of six orders of magnitude, but not a changeover in mechanism. The reported activation... 

Pentaammine(trifluoromethanesulfonato-0)iridium(III) trifluoromethanesul-fonate is a white powder that is air-stable provided it is not subjected to prolonged exposure to atmospheric moisture. The complex may be kept for many months in a desiccator over a suitable drying agent (silica gel, CaCL) without any noticeable decomposition. The aquation rate constant for the complex is 2.6 x 10 sec" at 25° (0.1 M CF3SO3H), and at elevated temperatures (60-80°) the rate of substitution is quite rapid in both aqueous and nonaqueous solvents. When dissolved in poorly coordinating solvents such as acetone or tetrahydrothiophene 1,1-dioxide (sulfolane), the solvent-substituted species are themselves comparatively labile, and many substitution reactions may be performed in these solvents. " ... 

Table 6.1. Kinetic Parameters and Aquation Rate Constants for [CrL(NH3)s] Ions... 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the... 

Rates and activation parameters associated with the values of ki, the independently determined aquation rate constants (fcaq), and the base-hydrolysis rate constants ( 2) are collected in Table 22. The activation parameters associated with the ki and k ci values are the same within experimental error. The hydroxide-ion-independent pathways lead to virtually complete trans- cis isomerization when X = Cl or Br, and to ca. 50% isomerization when X = I. The relatively small contribution of the k term in equation (16) (Jk lkx ca. 1 dm mol ) arises from the lower values of tsS k — A5" ( i) than are observed for cw-[Rh(en)2(OH)X]+ or [Rh(NHa)6X] +. Values of AA (k2) and lie at the upper end of an isokinetic plot that includes data for... 

Rate data are reported for the acid-catalysed aquation of the [M(NH3)5COal+ ions (M = Rh or Ir ), as well as the rates of formation of these carbonato-com-plexes from [M(NH3)50H] + and carbon dioxide. At 298.1 K and p = 0.5 mol 1 , the aquation rate constants are 1.13 and 1.45 s and the formation rate constants 470 and 5901 mol S for the rhodium(m) and iridium(m) complexes respectively. The close similarity of these rate constants for both metal ions indicates carbon-oxygen bond fission for the aquation reactions. 

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 . ... 

The dinuclear complex [(H3N)5Ru(/i-N2)Ru(NH3)5] has an aquation rate constant >10 s at room temperature, much faster than that for [Ru(NH3)5(N2)]. The tetranuclear cation [Ru4(OH)4] undergoes depolymerization, presumably giving initially [Ru2(OH)2], much more slowly. At 298 K the depolymerization rate constant is 2.5xl0 s = 63kJmorO. Linkage isomerization of complexes [Ru(NCS) -(SCN)6 ] n = 1-4), produced by reacting ruthenium trichloride with potassium thiocyanate in aqueous solution, under various conditions has been discussed in qualitative terms. ... 

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). ... 

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