Oxoanions transition elements

This chapter consists of a description of the ions formed in aqueous solutions by the transition elements - the d-block elements - and a discussion of the variations of their redox properties across the Periodic Table from Group 3 to Group 12. There is particular emphasis on the first transition series from scandium to zinc in the fourth period, with summaries of the solution chemistry of the second (Y to Cd) and third (Lu to Hg) series. The d-block ions in solution are those restricted solely to aqua complexes of cations, e.g. [Fe(H20)f,]" +, and the various oxocalions and oxoanions formed, e.g. V02+ and MnCXj". Oxidation states that are not well characterized are omitted or referred to as such. 

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. 

Table 7.3 Examples of oxocalions and oxoanions of the transition elements in acidic and alkaline solutions...

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. 

The ionic forms of transition elements in aqueous solution were described and the variation front cations to oxoanions was discussed. 

Figure 22.5 Aqueous oxoanions of transition elements. A, Often, a given transition eiement has muitipie oxidation states. Here, Mn is shown in the +2 (Mn -, /eft), the +6 (Mn04 , middle), and the +7 states (MnO ", right). B, The highest possible oxidation state equals the group number in these oxoanions V04 (/eft), Cr207 (middle), and Mn04 (right).

Figure 22.5 Aqueous oxoanions of transition elements. A, Mn has common +2...

The maximum oxidation state is generally found in compounds of the transition elements with very electronegative elements, such as F and O (in oxides and oxoanions). 

The transition from positive ions with low oxidation states, via insoluble oxides with intermediate oxidation states, to oxoanions with high oxidation states, is caused by the competition between ionization energies, lattice enthalpies and enthalpies of hydration, similar to the discussion of the variations of ionic forms of the p-block elements given in Section 6.1. Further discussion occurs in Section 7.5.3. 

This description of the main group ions of the first three columns of the periodic table shows that with the exception of alkali metal ions all these ions are adsorbed specifically the same is true of other ions, especially the heavy metal ions, which generally are of greatest practical interest. The other main group elements form anions or oxoanions directly. In the case of equal valence, the transition metal ions resemble each other more than the main group elements do, so more generalization should be possible. 

UV-vis spectroscopy shows that the doping elements present in the form of oxoanions. The absorption bands are due to ligand-metal charge transition. In all cases UV bands were found to correspond to polyoxoanions M Oy ". Fig. 4 gives the exemple of Mb doped samples which demonstrate a band characteristic for a niobate compound compared with the bulk oxide. 

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