Homoatomic Polyanions of the Early p-Block Elements

Liquid Ammonia: More than an Innocent Solvent for Zintl Anions

Liquid ammonia as the original solvent for Zintl anions has been replaced by easier to handle or more versatile solvents in most recent Zintl chemistry. However, methodological advances have made it possible to structurally investigate the anions in ammoniate crystals via crystallography or in the solutions themselves via nuclear magnetic resonance. While in some cases liquid ammonia acts as an innocent solvent, it also provides different possibilities of direct involvement in chemical reactions. In addition to simple dissolution without changes to the anions observed in the solid starting materials, protonation of the anion, incongruent dissolution involving redox processes, and further oxidation and reduction products have been observed. The use of the solvent liquid ammonia under ambient pressure is limited to low temperatures, which in turn allows the monitoring of kinetically stabilized species, some of which cannot be accessed at higher temperatures. In this work, the available literature reports are summarized or referenced, and compounds that have been characterized as new ammoniate crystals are presented and contextualized. Innocent dissolution is observed for clusters involved in K2.9Rb5.1[Si4][Si9]·15NH3, Cs4Sn9·12NH3, Cs4Pb9·5NH3, and [Rb@[18]crown-6]2[Rb@[2.2.2]crypt]Rb[Ge9]·4NH3. Formal protonation of [Ge4]4- results in the crystallization of [Na@[2.2.2]crypt]2[H2Ge4]·3NH3. Tt52- (Tt = Sn or Pb) and HSi93- cannot be accessed in a binary solid state material but can be crystallized in co-crystals of PPh3 in [Rb@[2.2.2]crypt]2[Sn5][PPh3]2·NH3, [Rb@[2.2.2]crypt]2[Pb5][PPh3]2·NH3, and [K@[2.2.2]crypt]3[HSi9][PPh3]·5NH3.

Structural Uniqueness of the [Nb(As5)2]5- Cluster in the Zintl Phase Cs5NbAs10

The structure of the novel Zintl phase, Cs₅NbAs₁₀, is reported for the first time. This compound crystallizes in the monoclinic P2₁/c space group (no. 14) with eight formula units per cell. The structure represents a unique atomic arrangement, constituting a new structure type with Wyckoff sequence e32. The most important structural element is the unprecedented [Nb(As₅)₂]^5- cluster anion, formed by a Nb atom enclosed between two As₅ rings. These nonaromatic cyclic species, formally [As₅]^5-, adopt an envelope conformation similar to that of cyclopentane. To date, it is only the second example of an [As₅]^5- ring with this conformation, reported in an inorganic solid-state compound. The bonding characteristics of the [Nb(As₅)₂]^5- cluster and the [As₅]^5- rings are thoroughly investigated using first-principles methods and discussed. Electronic band structure calculations on Cs₅NbAs₁₀ suggest that this compound is a semiconductor with an estimated band gap of ca. 1.4 eV.