Stability of oxidation states

Stannate(II) ions are powerful reducing agents. Since, for tin, the stability of oxidation state -b4 is greater than that of oxidation state -b2, tin(II) always has reducing properties, but these are greater in alkaline conditions than in acid (an example of the effect of pH on the redox potential, p. 101). 

For many years, the chemistry of silver and gold was believed to be more similar than is now known to be the case [1-10]. In the Cu-Ag-Au triad, the stability of oxidation states does not follow the usual trend of increasingly stable high oxidation state on descending the group for copper, the +2 state is the most important, for silver it is the +1 state and, though oxidation states between -1 and +7 are claimed, for gold it is the +1 and +3 states that dominate its chemistry. The types of compound are summarized in Table 4.1. 

The Stabilization of Oxidation States of the Transition Metals R. S. Nyholm and M. L. Tobe... 

As a more complex example, we examine the stability of oxidation states of aqueous sulfur as a function of pH. This exercise will bring out the treatment of thermodynamically unstable species and the change of sulfur speciation with pH. 

Other applications of emfs include the prediction of thermodynamically possible redox reactions [e.g., will Sn4+oxidize Fe2 to Fe3+ ] and the stabilization of oxidation states through the formation of complexes. The former is a straightforward application of thermodynamics and will not be discussed further here. The second is of great importance. It was introduced in Chapter 11 and will be discussed further below. 

Describe the trend in the stability of oxidation states moving down a group in the d block (for example, from chromium to molybdenum to tungsten). 

Apart from its own susceptibility to oxidation or reduction, a solvent can affect redox equilibria by modifying the relative stabilities of oxidation states of solutes. Thus Cu+ is unstable in aqueous solution to disproportionation (Section 5.4) but it is quite stable in acetonitrile. This arises from the relative magnitudes of the solvation energies and entropies of Cu+ and Cu2+ in the different solvents. In ammonia, cobalt(III) is much more stable relative to cobalt(II) than in water. The... 

Thus, relativistic effects define continuation of the trends in the IP, EA, bonding and stabilities of oxidation states in going over to the 6d elements, while the non-relativistic description of these properties would give opposite and, therefore, wrong trends. 

The knowledge of the relative stability of oxidation states, i.e., redox potentials, is very important for a chemical application. Trends in the stability of various oxidation states of the very heavy elements were predicted earlier on the basis of atomic relativistic DF and DS calculations in combination with some models based on a Born-Haber cycle (see [12]). The conclusions were, however, not always unanimous and varied depending on the model. Later, this topic received a more detailed consideration... 

Extensive DFT and PP calculations have permitted the establishment of important trends in chemical bonding, stabilities of oxidation states, crystal-field and SO effects, complexing ability and other properties of the heaviest elements, as well as the role and magnitude of relativistic effects. It was shown that relativistic effects play a dominant role in the electronic structures of the elements of the 7 row and heavier, so that relativistic calculations in the region of the heaviest elements are indispensable. Straight-forward extrapolations of properties from lighter congeners may result in erroneous predictions. The molecular DFT calculations in combination with some physico-chemical models were successful in the application to systems and processes studied experimentally such as adsorption and extraction. For theoretical studies of adsorption processes on the quantum-mechanical level, embedded cluster calculations are under way. RECP were mostly applied to open-shell compounds at the end of the 6d series and the 7p series. Very accurate fully relativistic DFB ab initio methods were used for calculations of the electronic structures of model systems to study relativistic and correlation effects. These methods still need further development, as well as powerful supercomputers to be applied to heavy element systems in a routine manner. Presently, the RECP and DFT methods and their combination are the best way to study the theoretical chemistry of the heaviest elements. 

With iron the trends already noted in the relative stabilities of oxidation states continue, except that there is now no compound or chemically important circumstance in which the oxidation state is equal to the total number of valence shell electrons, which in this case is eight. The highest oxidation state known is VI, and it is rare. The only oxidation states of importance in the ordinary aqueous and related chemistry of iron are II and III. The oxidation states and stereochemistries are given in Table 17-E-l. 

The complexing tendencies decrease, on the whole, in the same order as the hydrolytic tendencies. The formation of complexes shifts the oxidation potentials, sometimes influencing the relative stabilities of oxidation states thus the formation of sulfate complexes of Np4+ and Np02+ is strong enough to cause disproportionation of Np02. 

Smith, D. W. Stability Index Diagrams Pictorial Representations of the Relative Stabilities of Oxidation States for Metallic Elements, J. Chem. Educ. 1996, 73, 1099-1102. 

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