Transition metal ions equilibria

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Equilibrium considerations other than those of binding are those of oxidation/reduction potentials to which we drew attention in Section 1.14 considering the elements in the sea. Inside cells certain oxidation/reductions also equilibrate rapidly, especially those of transition metal ions with thiols and -S-S- bonds, while most non-metal oxidation/reduction changes between C/H/N/O compounds are slow and kinetically controlled (see Chapter 2). In the case of fast redox reactions oxidation/reduction potentials are fixed constants. 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions. 

The high affinity of Cu(II) for hydroxide results in the formation of higher hydroxy species so that Cu(II) redissolves at very high pH (i.e. it is amphoteric). This permits electrochemical studies to be conducted on solvated Cu(II) in aqueous solution under high pH conditions that are not feasible for most other divalent transition metal ions [165]. McDowell and Johnston [166] measured the increasing solubility of CuO in KOH solutions and interpreted their data in terms of the formation of both Cu(OH)3 and Cu(OH)4 for which they report the following equilibrium constant ... 

Inorganic radicals and transition metal ions typically exhibit broad lines, and hence diminished sensitivity in the EPR method. Consequently, when dealing with small quantities of paramagnetic material, it is often more difficult to detect inorganic species. Several important studies have been reported, however. Kastening s study [64] of the reduction of S02 in dimethylformamide showed that an equilibrium was established between the SO radical ion and the dimer S20, ... 

PBDA and BDA with some transition metal ions are shown in Table 1. For the polymer ligand, the complexation process appears to proceed in two steps a) accumulation of M2 + ions into the PBDA domain (pre-equilibrium process) due to electrostatic interaction of the anionic polymer backbone of PBDA with M2+,... 

Aminolevulinic acid, an element in the biosynthesis of heme proteins, may be overexpressed under certain pathological conditions and its accumulation has been correlated with some hepatitic cancers. It is in equilibrium with its enol form and can complex transition-metal ions. In the presence of 02, this may lead to the formation of OH and hence in the presence of DNA to DNA damage which is enhanced in the presence of ferritin (Douki et al. 1998 di Mascio et al. 2000). Superoxide radicals have been assumed to be intermediates in these reactions. Mechanistically, the formation of 02 is certainly very complex, because even in the case of the Fe(II)-EDTA-complex the reduction potential is not low enough to reduce O2 by simple one-electron donation. 

Again, in the oxidation of transition-metal ions, adducts have been established as intermediates [e.g., reaction (35) O Neill and Schulte-Frohlinde 1975 Asmus et al. 1978 for the equilibrium of Tl2+ and OH, see Schwarz and Dodson 1984],... 

NH3)5]"+ (n = 4, 5, or 6) and [(bipy)2ClRu(pyz)RuCl(bipy)2]"4 (n = 2, 3, or 4).50 These studies reveal equivalent metal sites even in the mixed-valency Run-Ru,u species, despite the speed with which ESCA monitors electron distribution (10 17 s). Another investigation shows that the complex [Ru(NH3)5(pyz)]2+ can associate with several aqueous first-row transition-metal ions to form pyrazine-bridged dinuclear complexes.51 Equilibrium formation constants with Ni", Cu , and Zn11 were measured spectroscopically. 

If two phases are in equilibrium with one another at some temperature and pressure, a transition metal ion will be distributed between the phases in such a way as to minimize the free energy of the two-phase assemblage. This should generally result in the transition element being concentrated in the phase giving largest crystal field stabilization energy. 

The oxidation of NO and N02 is of industrial importance for cleaning combustion-flue gases. Transition metal ion-exchanged zeolites have been shown (51) to be highly active catalysts for this reaction. The relative activities are shown in Fig. 10, from which it can be seen that equilibrium conversions of NO to N02 can be achieved with Cu2 + X at reaction temperatures as low as 350"C. Kinetic studies showed that the reaction rates were fractional order in both NO and 02. The following reaction mechanism was therefore proposed, for example, with Cu2 + X,... 

The kinetic approach is of restricted utility because it is applicable to (i) slow reactions, (ii) some transition metal ions, (iii) the role played by the electronic structure of the central metal ion. The equilibrium approach is more convenient than the kinetic approach and hence discussed here in a detailed manner. In general when a metal M complexes with a ligand A and forms complexes of the type MA, MAj. .. MA/v we may write for the total concentrations of M and A as... 

Crown ethers have particularly large complexing constants for alkali metals equilibrium constants for cyclohexyl-crown-6, for example, are in the order K+ > Rb+ > Cs+ > Na+ > Li+. The cryptates have high complexing ability especially for M2+ ions and will render even BaS04 soluble. They also have good complexing ability for transition metal ions (e.g., for lanthanides). 

The relative rates of reaction of the nucleic acid bases with heavy transition metal ions at neutral pH are in the same order as the relative nucleophilicites of the bases, that is G > A > C > U or T. This order parallels the relative rates of reactions for cA-[(NH3)2Pt(OH2)2] (see Figure 9), while the equilibrium constants for the same reactions are very close in magnitude. In contrast, HsCHgOH, which is more labile to substitution, nndergoes more favorable binding with deprotonation at N-3 of thymine residues in nucleic acids. Thus the relative facilities of individual reactions can lead to differences in initial product formation (kinetic control). Subsequent changes in the metal-nucleic acid complexes can be nnder kinetic or thermodynamic control. 

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