Rubidium suboxides

In addition to the above oxides M2O, M2O2, M4O6, MO2 and MO3 in which the alkali metal has the constant oxidation state 4-1, rubidium and caesium also form suboxides in which the formal oxidation state of the metal is considerably lower. Some of these intriguing compounds have been known since the turn of the century but only recently have their structures been elucidated by single crystal X-ray analysis. Partial oxidation of Rb at low temperatures gives RbeO which decomposes above —7.3°C to give copper-coloured metallic crystals of Rb902 ... 

Other examples of compounds of this type are the suboxides of rubidium and caesium, made by fusing the normal oxides, Rb20 and CS2O, with the parent metals. These are lustrous and have good electrical conductivities, diminishing with increase of temperature. There are several for each metal, examples being Rb902, which is copper coloured, and CsyO, which is bronze. 

The presence of any electron donor able to stabilize the cation will therefore favor cluster disruption. Such limiting conditions make it very difficult to find an adequate medium for the stabilization of such kinds of clusters. Solvent as well as counterions must be highly inert indeed, both as oxidation agent and as a Lewis base. Although the severity of such limiting conditions increases with increasing atomic weight of the metals, it has been possible to achieve the conditions under which clusters of the elements rubidium and cesium can exist, namely as the suboxide to be described in this Section. 

The formation of homonuclear bonding between alkali-metal atoms which characterizes the formation of cluster species implies that these elements are in an intermediate oxidation state between 0 and 4- 1. This condition appears to be met by the suboxides of rubidium and cesium. 

Selected characteristics of the suboxides of rubidium and cesium are summarized in Table 4.4. 

In the metal-rich suboxides the metal is concentrated in purely metallic regions. In Fig. 4.8, a scheme of the structure of the phase Rb60 is reproduced. There the Rb902 clusters are arranged in layers that alternate with others of metallic rubidium atoms. In the case of the CS7O as illustrated in Fig. 4.9 the CS11O3 clusters are arranged to form columns which are in turn surrounded by purely metallic cesium atoms. 

In contrast to transition-metal molecular clusters, the alkali-metal suboxides are stable only in the solid state. As described in Table 4.4, these clusters decompose at temperatures rather below the melting point of the metals. The stability of these species appears to be relatively precarious. It is very probable that the stabilization of this class of extreme electron-deficient compounds is possible only at relatively low temperature and in strong reducting media such as the alkali-metals rubidium or cesium. 

Controlled oxidation of mixtures of both rubidium and cesium metals leads under equilibrium conditions only to the cesium clusters CsnOa in the form of the compounds [CsnOaJCsio-xRbx, [CsnOajRbv-xCSx, or [CsiiOajCsi-xRbx. Only after the consumption of all the cesium can the rubidium partially replace the cesium in the clusters. This feature indicates the presence of equilibria between actual chemical species with different relative thermodynamic stabilities. The ionization energy of cesium is lower than that of rubidium. This feature appears to be determinant in the relative stability of the suboxides, higher for CsuOa than for Rb902, deduced from the experiments discussed above. 

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