Zintl anions are negatively charged clusters of main group elements, such as [Sn₉]⁴⁻, [Pb₉]⁴⁻, and [Sb₇]³⁻. Many clusters of the type [E₉]^4–  (E = Ge, Sn, Pb) have been investigated. They can be oxidized to give a large variety of cage-like structures, coupled clusters such as [(Ge₉)_n]^m–, and new element allotropes. However, there are few examples of clusters with group 14 elements and ten atoms which are not stabilized by other metals (either inside the cage or bound to the periphery).

Thomas F. Fässler, Technical University of Munich, Germany, and colleagues have synthesized and characterized unbound and empty closo-[Ge₁₀]^2- anions for the first time. The team synthesized crystals of [Rb(222-crypt)]₂[Ge₁₀](en)_1.5 (en = ethylenediamine) by preparing a solution of Rb₄Ge₉ and 7-amino-1-(trimethylsilyl)-5-aza-hepta-3-en-1-yne in ethylenediamine and layering the solution with cryptand[2.2.2] in toluene. After two weeks, the product formed as dark purple, pillar-shaped crystals in yields of 10–20 %.

The researchers characterized the compound using X-ray crystallography, Raman spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). They found that [Ge₁₀]^2- has a D_4d-symmetric closo-structure (a bi-capped square antiprism, pictured). The team explains the formation of the [Ge₁₀]^2- product rather than [Ge₉]^4- by the fact that the latter is more reactive and forms functionalized [Ge₉R]^3– species, which do not crystallize under the reaction conditions.

• Synthesis and characterization of pristine closo-[Ge₁₀]^2–,
  Manuel M. Bentlohner, Christina Fischer, Thomas F. Fässler,
  Chem. Commun. 2016.
  DOI: 10.1039/C6CC04143D