Metal oxides

Key points: The oxides formed by metals include the basic oxides with high oxygen coordination number that are formed with most M⁺ and M⁺² ions. Oxides of metals in intermediate oxidation states often have more complex structures and are amphoteric. Metal peroxides and superoxides are formed between O₂ and alkali metals and alkaline earth metals. Ter minal E O linkages and E-O-E bridges are common with nonmetals and with metals in high oxidation states. The O₂ molecule readily removes electrons from metals to form a variety of metal oxides containing the anions O₂ (oxide), O₂ (superoxide), and O₂^-2 (peroxide). Even though the exist ence of O^-2 can be rationalized in terms of a closed-shell noble gas electron configuration, the formation of O^-2 (g) from O₂ (g) is highly endothermic, and the ion is stabilized in the solid state by its strong Coulombic interaction with the surrounding cations. Alkali metals and alkaline earth metals often form peroxides or superoxides (Sections 11.8 and 12.8), but peroxides and superoxides of other metals are rare. Among the metals, only some of the noble metals do not form thermodynamically stable ox ides. However, even where no bulk oxide phase is formed, an atomically clean metal surface (which can be prepared only in an ultrahigh vacuum) is quickly covered with a surface layer of oxide when it is exposed to traces of oxygen. Structural trends in the metal oxides are not readily summa rized but for oxides in which the metal has oxidation number +1, +2, or +3, the O^-2 ion is generally in a site of high coordination number:

M(I): M₂O oxides often have a rutile or antifluorite structure (6,3)- and (8,4)-coordination, respectively. M(II): MO oxides usually have the rock-salt structure (6,6)-coordination, M(III): M₂O₃ oxides often have (6,4)-coordination.

At the other extreme, _MO4 compounds are molecular: the tetra hedral compound osmium tetroxide, OsO₄, is an example. The structures of the oxides with metals in high oxidation states and the oxides of nonmetallic elements often have multiple bond character. Deviations from these simple structures are common with p-block metals, where the less symmetric packing of O^-2 ions around the metal can often be rationalized in terms of the existence of a stereochemically active lone pair, as in PbO (Section 14.11). Another common structural motif for nonmetals and some metals in high oxidation states is a bridging oxygen atom, as in E-O-E, in angular and linear structures.

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مواضيع ذات صلة

Occurrence, recovery, and uses

The interhalogens

Oxides of selenium and tellurium

Metal sulfides, selenides, tellurides, and polonides

Halides

Hydrides of sulfur, selenium, and tellurium

Thermochemical correlations

Magnetic measurements

Octahedral complexes

Sulfide complexes

Disulfides

Polyoxometallates