Transition elements
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Why are oxides of transition metals with a high oxidation state generally acidic?
Ulex replies
It is always difficult to know at what level to answer a question which begins ‘Why…. ?’ It is almost always a matter of linking the phenomenon under discussion with other related phenomena, and there is often more than one way of going about it. I would argue as follows.
According to Fajans’ rules, when a cation would, if it existed, be both small and highly charged, the bonding becomes covalent rather than purely ionic. This is because, given the two possibilities, the covalent bonding is more energetically favourable than the ionic bonding.
This means that the formation of, for example, manganese(VII) ions, hypothetically Mn7+, would be energetically unfavourable. (Reasonable, considering the massive ionisation energies involved). Manganese(VII) oxide, Mn2O7, would therefore be covalent rather than ionic. When an ionic oxide such as MgO reacts with an acid, the Mg2+ ions which it contains merely hydrate and go into solution.
This option is not available to manganese(VII) oxide. Like other covalent oxides, it can react with alkalis to form salts in which the manganese is covalently bonded. Hence the existence of KMnO4 which contains the ion MnO4-. Such a reaction involves the breaking of some covalent bonds and the formation of others. This, and other similar, types of reaction are energetically favourable.
Covalent oxides are characteristic, generally of non-metal elements. Non-metal oxides are generally acidic.
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updated: 19 March 2006
