Oxidation States of the Transition Elements

The oxidation state concept is best used for the transition elements, and Table S.4 shows the oxidation states exhibited by the three series of transition elements in their many and varied compounds. The oxidation states shown in bold red are those that are the most stable in acidic aqueous solution in the presence of air. Any oxygen present would possibly oxidize an otherwise stable state, e.g. Fe ions are the most stable state of iron in aqueous solution in the absence of dioxygen which would (slowly) oxidize the Fe ions to Fe. The states that are most stable in the absence of air are shown in italic red, if different. As the oxidation state of any element increases, there is an increasing tendency for the compounds of that element to be covalent. In low positive oxidation states the compounds are ionic and have high melting points, but as the oxidation state increases the compounds tend to be covalent and molecular and so have low melting points. 

The pattern of stable oxidation states in aqueous solution is different from that of the solid compounds of the transition elements. As the oxidation state of an element increases, there is a tendency, particularly in the later groups, for the compound to act as an oxidizing agent. If the compound is dissolved in water, the compound oxidizes the water to dioxygen and is consequently reduced to a lower oxidation state, one which does not have the capacity to oxidize water and which is then stable. 

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The higher oxidation states of the transition elements may be considered to be hydrolysis products of hypothetical more highly charged cations in which the central metal ion is sufficiently electronegative to be able to participate in covalent bonding. For example, the hypothetical Mn7 + ion interacts with water to give an oxoanion, the manganate(VII) ion ... 

Examples of oxidation states of the transition elements that exist in aqueous solutions as oxocations or oxoanions are given in Table 7.3. Oxoanions tend to predominate over oxocations as the oxidation state of the central metal ion increases. 

The insolubility of the hydroxides of the lower oxidation states of the transition elements is the reason for the general lack of aqueous chemistry in alkaline solutions. The higher oxidation states of the elements take part in covalency to produce oxoanions and persist even in alkaline conditions, and allow their solubility. 

Mercury-transition metal bonds have been described for all members of Groups V-VIII of the transition series except, apparently, technetium. They commonly involve a low oxidation state of the transition element and are particularly numerous for the chromium, iron and cobalt families.1 In addition, mercury-titanium bonded species have been postulated as unstable reaction intermediates.2... 

Each of the transition elements tends to have several oxidation numbers. Thus iron forms one series of compounds with oxidation number -f-2 (ferrous compounds) and another series with oxidation number - -S (ferric compounds). For chromium the principal oxidation numbers are -f 3 and -f 6, and for manganese they are -f 2 and +7. It would be of great value to chemistry if a simple and reliable theory of the oxidation states of the transition elements were to be developed but this has not yet been done. 

We are honored to be able to dedicate this paper to Professor Wilhelm Klemm. Professor Klemm s pioneering work on high oxidation states of the transition elements and his continuing interest in such work suggested to us that description of the synthesis and properties of Au(V) salts would be an appropriate contribution in his honor. 

Table 5.4 Oxidation states of the transition elements those shown in bold red are most stable in aqueous solution those in italic red are most stable in the absence of dioxygen...

The general variability of the oxidation states of the transition elements is explicable in terms of the closeness of values of successive ionization energies and, for any coordination number, the existence of odd numbers of anti-bonding electrons is not unusual. Such anti-bonding electrons possess almost the same energies that they do in the isolated oxidation state, and do not destabilize the systems sufficiently to rule out their existence. In the latter halves of the transition series, successive ionization energies increase and the attainment of hi er oxidation states is less apparent. 

Across the periods there is a transition from ionic to covalent compounds, with elements showing their expected valencies/ oxidation states, and with some of the p-block elements showing the inert pair effect and/or hypervalency. The transition elements exhibit a wide range of oxidation states in both fluorides and oxides, with the oxides varying from basic to acidic as the oxidation state of the transition element increases. 

Table 24.2 Common oxidation states of the transition elements.

The higher oxidation states of the transition elements are found in complex ions or in compounds formed with oxygen or fluorine. Common examples are the chromate(VI) ion, CrO ", and the manganate(VII) ion, MnO -. 

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