Spontaneous aquation

If the spontaneous disproportionation of Brl" is accelerated when Co(II) is present (in concentrations comparable to those used in the Bi2 /Bt2 experiments) then (2) is the favored fast step. This step is inner-sphere if Co(lll) — Br is detected and its aquation to Co(lll) —H2O noted. 

Configuration Changes Accompanying Spontaneous and Assisted Aquation... 

The products of reduction of Fe(NCS)2+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H20)4]+, which contains both bland S-bonded thiocyanate ligands.641 These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr—SCN— Cr—NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(III) complexes for comparison are listed in Table 68. From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between Nj and NO. 

Spontaneous Aquation Rates of Pentaammineosmium(III/II) and Analogous Pentaammineruthenium(III/II) Complexes"... 

Table 5 Rate Constants, Activation Parameters and Steric Courses for the Spontaneous Aquation of cis- and...

A S oh = + 38 + 6JK mol .5SS For both the spontaneous and base-catalyzed paths, the rates of isomerization for the three pentaammines follow the order Rh > Co > Ir for the base-catalyzed path the reaction rates are of the order 278 217 1, while for the spontaneous path (ks), the relative rate constants are 32 2.3 1. The greater reactivity of the Rh complex, attributed to a lower AH for both the spontaneous and base-catalyzed paths, contrasts sharply with the order generally observed for the aquation of d6 pentaammines. For example, for M = Co, Rh and Ir, the aquation of [M(NFI3)5Br]2+ in acidic solution occurs with relative rate constants of 4 1 0.001.587 The ordering is not always Co > Rh P Ir, however, as the rates of water exchange in aquapentaammine complexes in the Rh triad have relative rates of 0.53 1 0.0059,559 and the relative rates of acid aquation of the [M(NH3)5(C03)r ions are 1 1 0.02.518... 

It is well documented that the aquated form of cisplatin localizes within the mitochondria, and that the inhibition of respiration and spontaneous release of calcium ions into the... 

Recent stereochemical studies by Loeliger and Taube (85) support the view that the departing X may have an effect on the steric course of aquation reactions in these systems. For example, the spontaneous aquation of induced aquation with the addition of Hg + yields 28% cis-and 72% rapid reaction of trans-[Co (en) 2X301]+ with HNO2 to liberate N2 and N2O in aqueous solution 5delds [Co(en)2(H20)Cl]2+ in the same isomeric quantities as for the induced aquation with added Hg +. Since these two different rapid reactions give the same products, it is assumed that the products are formed from the same 5-coordinated intermediate, as represented by Equation 2. 

It then would appear that the spontaneous aquation of product ratio, must not involve the same 5-coordinated species. A plausible interpretation is that in the normal aquation process the leaving Cl is still near enough to the pseudo 5-coordinated species to affect the position of attack by the entering H2O. In relation to Equation 2, spontaneous aquation can be illustrated by Equation 3. 

At 25°C unless indicated kn and refer to first-order rate constants for spontaneous and metal-assisted aquation paths, respectively. 

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. 

The observed effects are particularly dependent on the central metal ion. For example, the rate-enhancing effect of polyethylenesulfonate on spontaneous aquation of [CrlNHslsBr] " is very small compared with the effect on the cobalt(III) analog (140). Given the apparent... 

NOJ(aqf). This includes solid ammonium nitrate, the water in which it dissolves, and the aquated ions that are the products of the dissolution process. The surroundings include the flask or beaker in which the system is held, the air above the system, and other neighboring materials. The dissolution of ammonium nitrate is spontaneous after any physical separation (such as a glass wall or a space of air) between the water and the ammonium nitrate has been removed. 

An example of a nearly pure first-order irreversible reaction is the decay of radioactive isotopes. These reactions depend on instabilities within the nucleus rather than the electronic configuration of the atom and are thus totally uninfluenced by the surroundings. The reaction rate is dependent on the concentration of the isotope and the probability of spontaneous decay. An important example in aquatic systems is the decay of carbon-14 (Fig. 9.5), which is formed in the upper atmosphere by interaction between cosmic ray particles and nitrogen atoms. is then mixed into the lower atmosphere and... 

In a series of publications, Stevenson et al. have demonstrated that the enthalpies of generation of polyacene dianions can be obtained by calorimetric measurements. Compared to their calculated instability in the gas-phase 70), THF solutions of polycyclic dianions are thermodynamically and kinetically stable as evidenced by their spontaneous formation and persistence. The solvation energy of the dianions plus that of two cations must overcome the repulsive interaction of the charges. This aspect has been demonstrated by Stevenson in his studies on the cyclooctatetraene dianion 70). The dianion sodium salts were prepared in THF in thin-walled glass bulbs and the bulbs were crushed under water in a calorimeter system. The heat of aquation of the solvent is taken into account and thus, the net change in the heat content of the calorimeter vs. the millimoles of dianion salt is obtained. The plots are linear and the slopes represent the enthalpies of the following reaction (Eq. 20) ... 

Rehulka, J. Spontaneous nephroblastoma in a hatchery rainbow trout Oncorhynchus mykiss. Dis. Aquat. Org. 14 75-79, 1992. 

Winn, R.N., R.J. Van Beneden and J.G. Burkhait. Transfer, methylation and spontaneous mutation frequency of PHI-X174AM3CS70 sequences in medaka Oryzias latipes) and mummichog Fundulus heteroclitus) — implications for gene transfer and environmental mutagenesis in aquatic species. Mar. Environ. Res. 40 247-265, 1995. 

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