General Redox Chemistry of the d-Block Elements

Although there is no space to develop a detailed discussion of the solubilities of compounds of the transition elements, the general insolubility of their + 2 and + 3 hydroxides is important. The rationale underlying their insolubility can be summarized (i) the hydroxide ion is relatively small (152 pm ionic radius) and the ions of the +2 and +3 transition metals assume a similar size if their radii are increased by 60-80 pm, and (ii) the enthalpy of hydration of the hydroxide ion (—519 kJ mol ) is sufficiently negative to represent a reasonable degree of competition with the metal ions for the available water molecules, thus preventing the metal ions from becoming fully hydrated. Such effects combine to allow the lattice enthalpies of the hydroxides to become dominant. 

By contrast, the chlorides of the metal ions are soluble because the chloride ion (181 pm ionic radius) is considerably larger than the hydroxide ion, and its enthalpy of hydration ( — 359 kJ mol l) is less negative than that of OH. This allows the metal cations to exert more nearly their full effect on the solvent molecules, thus overcoming the lattice enthalpy terms, and this leads to their general solubility as chlorides. 

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. 

Group 3 of the Periodic Table consists of the elements scandium, yttrium and either lanthanum or lutetium, depending upon the preferred arrangement of the Table. Group 3 elements have the outer electronic configuration ns2 p, and invariably their solution chemistry is that of the + 3 state. In this text, treatment of both La and Lu is carried out in Chapter 8, which deals with the f-block elements. Lanthanum and lutetium represent the first and last members of the lanthanide series. 

The elements of the Group, including La and Lu, are powerful reducing agents, and their 4- 3/0 standard reduction potentials in I mol dm 3 acid solution are summarized by the Latimer diagrams shown in the margin. 

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