Alkali Metal Suboxide

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. 

Controlled additions of oxygen to rubidium or cesium lead to colored products with metallic properties when the metal is in excess. With the aid of sophisticated techniques for preparing, manipulating, and investigating these substances, detailed thermal studies of the products from these reactions have been carried out. 

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

Compound Structure Color Fusion or Disc.Temp. rc) Molar Volumes (cm mol ) (extrapolated at OK) Ref. 

Rb902 Rb902 clusters metallic copper-like platelets 40 387 1,2 

---

Simon, A. Structure and Bonding with Alkali Metal Suboxides. Vol. 36, pp. 81-127. 

Even the extremely electron-deficient alkali metals can form clusters when interstitial atoms contribute to their stabilization. Compounds of this kind are the alkali metal suboxides such as Rb902 it has two octahedra sharing a common face, and each is occupied by one O atom (Fig. 13.16). Flowever, the electron deficiency is so severe that metallic bonding is needed between the clusters. In a way, these compounds are metals, but not with single metal ions as in the pure metal Rb+e-, but with a constitution [Rb902]5+(e )5, essentially with ionic bonding in the cluster. 

The alkali metal suboxides are well understood in terms of formation, structure, and properties, yet we still face some unsolved problems that will also be addressed in the following. In contrast, the first suboxide of barium came to light only recently [24], and one has to see whether this compound is the tip of another iceberg, as turned out to be the case with the subnitrides of alkaline earth metals. 

Fig. 3.1-3. Crystal structures of the binary alkali metal suboxides (a) [Rb902], (b) [Csnn03], (c) [Rb902]Rb3(Rb60), (d) [Csn03]Csio(Cs70), and (e) [Csn03]Cs(Cs40). O atoms drawn as small spheres, for clarity only M-M contacts in the clusters outlined.

Fig. 3.1-4. Crystal structures of the ternary alkali metal suboxides (a) [Csn03]Rb, (b) [Csn03]Rb2, (c) [Csn03]Rb7.

The structural principle of combining salt-like bonded units with stoichiometric amounts of alkali metal in the structure of Ba2ONa is equivalent to that which characterizes the alkali metal suboxides as well as the subnitrides of alkaline earth metals, such that a variation at least according to a general formula [Ba20 Na% can be expected. Indeed, there are indications of the existence of further suboxides in... 

The structural principle of alkali metal suboxides has been unique for many years. Early attempts to synthesize cluster-type subnitrides of Ba resulted in the only compound, Ba2N, that is isotypic with Ca2N. Beyond these, the Sr analogue Sr2N, and most recently Sr4N3 [34], have been characterized. 

All suboxides and subnitrides described in the preceding sections are metallic. In the case of the alkali metal suboxides this property has been demonstrated by measurements of the electrical conductivity [58], CS7O, for example, exhibits a free electron like behavior in the temperature dependence of its resistivity rather similar to the element Cs itself. The characteristic colors of the alkali metal suboxides have been mentioned before, and spectroscopic investigations to be discussed in the following provide a more quantitative access to the metallic properties and the underlying chemical bonding. 

A Simon Structure and Bonding with Alkali Metal Suboxides... 

The bonding within the alkali metal suboxides is illustrated by comparing the interatomic distances in the compounds Rb9C>2 and CS11O3 with those in... 

Table 12.2.2. Structural data of alkali metal suboxides...

Discrete [BaeN] clusters are found in the crystal structure of (BaeN)Nai6, as shown in Fig. 12.6.4. (Ba6N)Nai6 crystallizes in space group Im3m with a = 1252.7 pm and Z = 2. The Na atoms are located between the cluster units, in a manner analogous to that in the low-valent alkali metal suboxides. 

The chemistry of the alkali metals suboxides such as RbeO, CS7O, Rb902, and CS11O3 has been reviewed by Simon and will not be discussed here. Rather, this section will concentrate on transition metal compounds where d electrons are utilized in bonding. 

IV. Chemical Bonding in the Stable Alkali Metal Suboxides.100... 

---