Sulphur Oxides and Oxoanions

Sulphur has a rich chemistry with oxygen. Sulphur forms strong double bonds to oxygen, with, in addition to the σ-interaction, both Opπ-Spπ and Opπ-Sdπ interactions occurring.

Oxides with sulphur in low oxidation states occur, but have very short lifetimes. These are SO, (SO)2 (which is a square with similar atoms at opposite corners) and S2O (which has the same structure as SO2, with one of the oxygen atoms replaced by sulphur).

The principal oxides of sulphur are sulphur dioxide, SO2, and sulphur trioxide, SO3. They both act as Lewis acids, with the sulphur atom being the electron pair acceptor (into a vacant S 3d orbital), and SO3 is a much stronger and harder acid.

Sulphur Dioxide, SO2

This is formed by direct combination of the elements. This has the bent, C2v, structure like water, with a S-O-S bond angle of 120o, and the Lewis picture of the structure can be seen as an equilibrium of two resonance forms, such that each atom has a completed octet.

Sulphur Trioxide, SO3:

When SO2 is burnt further, or when S is burnt in excess oxygen, SO3 is formed. This has the trigonal planarD3h, structure, again existing in resonance forms, again such that each atom has a completed octet. The Lewis acid nature of SO3 leads to its polymerizing under ambient conditions. It also acts a Lewis acid to other substances, provided that it does not oxidize them.

SO3 trimerises under ambient conditions to give (SO3)3, with the C3v structure.

Sulphur Oxoanions and Oxy-Acids

There are many sulphur oxoanions, some possessing just S-O bonds and others possessing both S-O and S-S bonds. The range of stoichiometries of the sulphur oxoanions leads to  a range of redox chemistry. Some of the common anions are shown in the table. Note that structures containing a S-S bond have the term thio- in their nomenclature.

Sulphite, SO32-:[OS(S) = +4] Sulphate, SO42-:[OS(S) = +6] Thiosulphite, S2O32-:[OS(S) = +2] Dithionite, S2O42-:[OS(S) = +3] Dithionate, S2O62-:[OS(S) = +5]

Polysulphur oxoanions also occur. These are polymeric in two ways: they can have bridging -O-S(O)2– or just -S- groups.

Polysulphates:bridge is -O-S(O)2 Polythionates:bridge is -S-

The reactions of these ions in solution often depends upon the pH. This can be demonstrated by the reactions of SO2 and SO32-. H2SO3 in the hydrated form of SO2, and so in basic solution SO2 exists as SO32-. SO2 in acidic solution acts as an oxidizing agent, whereas in basic solution SO32- is a reducing agent.

acidic solution E = +0.50 V
basic solution E = -0.94 V