Complexes standard reduction potentials

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

Consistent with this, even KI3 is rapidly decolorized in alkaline solution. The example is a salu-tory reminder of the influence of pH, solubility, and complex formation on the standard reduction potentials of many elements. 

The IrIV anion, [Ir(H20)Br5], oxidises ascorbic acid at 20.0 °C.51 This reaction is first order with respect to ascorbic acid concentration and first order with respect to the Irlv anion. Comparison of hexabromo-, hexachloro, aquopentachloro-, and di-aquotetracholoiridium(IV) reactions with ascorbic acid shows that replacing a halide ion with a water molecule increases the standard reduction potential of the IrIV complex and increases the rate of reaction. 

For a particular iron(III) oxidant, the rate constant (log kpe) for electron transfer is strongly correlated with the ionization potential Ip of the various alkylmetal donors in Figure 4 (left) (6). The same correlation extends to the oxidation of alkyl radicals, as shown in Figure 4 (right) (2). [The cause of the bend (curvature) in the correlation is described in a subsequent section.] Similarly, for a particular alkylmetal donor, the rate constant (log kpe) for electron transfer in eq 1 varies linearly with the standard reduction potentials E° of the series of iron(III) complexes FeL33+, with L = substituted phenanthroline ligands (6). 

The zinc complex is more difficult than the free Zn2+ to reduce as deduced from the more negative standard reduction potential ... 

An LDH with the approximate stoichiometry Mgo.3Co(II)o.6Co(III)o.2(OH)2 (N03)o.2 H2O has been synthesized by oxidation of Co(ll) using an am-moniacal solution and hydrothermal treatments vmder various O2 N2 atmospheres [176]. The ammoniacal solution plays a number of roles in the synthesis. Firstly, it provides a basic medium. Secondly, it acts as hgand by forming a complex [CoCNHsle] ", which facihtates oxidation of Co to [CoCNHsle] because of the low standard reduction potential (E°) ... 

Fig. 3. Correlation between the free energies of activation for the reactions given by Eq. (2) and the standard reduction potentials for the tris(diimine)metaKIII) complexes [cf. Eq. (4) ]. The data are from Table II.

The rate constant for the redox step, kr, is unlikely to reflect a simple electron transfer from the monodentate diimine ligand to the metal center because replacement of coordinated water with coordinated hydroxide would be expected to decrease the oxidizing power of the metal-(III) center. This is well documented by the standard reduction potentials of the aqua and hydroxo complexes in Table IV, and it would seem... 

Standard Reduction Potentials and Acid Dissociation Constants for Some AquapentakisiiminejRUthenium and -osmium Complexes at 25°Ca... 

TABLE 10.6 Standard Reduction Potentials of Copper Complexes in H20... 

There are very few reports in the literature concerning heterogeneous photocatalysis for uranium treatment in water. In our previous review, only one case of photocatalytic reaction on uranium salts was reported (Amadelli et al., 1991). Taking into account the standard reduction potentials, U(VI) can be photocatalytically reduced by Ti02 conduction band electrons to U(V) and then to U(IV) (E° = +0.16 V and +0.58 V, respectively, Bard et al., 1985). However, more reduced U(III) and U(0) forms cannot be generated because of very negative redox potentials (Bard et al., 1985). In addition, U(V) rapidly disproportionates to U(VI) and U(IV), and its chemistry is very complex (Selbin and Ortego, 1969). For example, uranyl... 

The ability of the fight actinides to access multiple oxidation states leads to rich, and, sometimes, complex electrochemistry. The standard reduction potentials at pH = 0 for each of the... 

Table 6 Standard reduction potentials (vs SHE) for some Co(III) complexes...

The Tl -Tl relationship is therefore a dominant feature of thallium chemistry. The standard reduction potentials at 25 °C and unit activity of H+ are TIVtI = —0.336 V, T1 /T1 = +0.72 V, and Tl /Tli = +1.25V. Estimates have also been made for the couples T1 /T1 = +0.33 V and Tl /Tl = 2.22 V. The generally valid limitations concerning the use of standard electrode potentials to predict the redox chemistry of real systems are especially important in the case of thallium factors such as complex formation in the presence of coordinating anions or neutral ligands and pH dependence due to hydrolysis do affect the actual or formal redox potentials. For example, redox potentials have been measmed for TICI/TICI3 =+0.77 V in IM HCl and T10H/T1(0H)3 = —0.05 V in alkaline soluhon. These formal potentials differ from the standard value for Tiin/Tii = +1.25 V. The difference can be attributed to the substanhal difference between the complex forming abilities of Tl and Tl , which will be discussed in detail later. The... 

The corrosion products of noble metals such as copper and silver are complex and affect the use of these metals as decorative materials. Under normal atmospheric conditions copper forms an external layer of greenish copper carbonate called patina. Silver tarnish is silver sulfide (Ag2S), which in thin layers gives the silver surface a richer appearance. Gold, with a positive standard reduction potential (1.50 volts), significantly larger than that for oxygen (1.23 volts), shows no appreciable corrosion in air. 

Reduction of the metal radicals. The anionic complexes CpM(CO)3" are well known species they are stable entities with 18 valence electrons. The standard reduction potential for the CpMo(CO)3 -CpMo(CO)3 couple is -0.08 V vs SSCE. The molybdenum radical is thus a mild oxidizing agent with suitable electron donors it can be reduced to the anion. For example, the radical oxidizes Fe(Ti -C5Meg)2 with a rate constant of 2.2 x 10 L moT s in acetonitrile at 23 °C. ... 

From their standard reduction potentials, in the range of 1.35 V versus NHE [37], complexes should not be able to oxidize methylbenzenes, the standard potentials of which exceed 1.75 V versus NHE [29]. The reaction, however, proceeds easily in the presence of a pyridine base, according to the following stoichiometry ... 

Knowing and understanding standard reduction potentials, E°, also helps determine whether a particular oxidation state will be stable and appropriate oxidants and reductants to use in a synthetic scheme. In reviewing the cobalt complex syntheses in Chapters 2 and 6, for example, complex ions are formed by oxidizing cobalt(II) salts to the more stable +3 state. 

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