Protonated Zintl-type clusters of group 14 elements (tetrels, E) are rare. This is in spite of the fact that these clusters, e.g., E₄^4–, are highly basic. Examples of E₄^4– ions as ligands in transition-metal complexes are also rare.

Thomas F. Fässler and colleagues, Technical University of Munich, Garching, German, have synthesized a diphenylzinc complex of the protonated tetrahedral tetrel cluster anion [Ge₄]^4–, i.e., [HGe₄ZnPh₂]^3– (pictured). The team used crown ethers to capture the counterions and, thus, improve solubility. They combined K₆Rb₆Ge₁₇, ZnPh₂, and [18]crown-6 in liquid ammonia and obtained [K([18]crown-6)][Rb([18]crown-6)]₂[HGe₄ZnPh₂]·8 NH₃. The four germanium ions (pictured in green) form a distorted tetrahedron, which is protonated on one side and bound to zinc (pictured in violet) on the other.

A single-crystal structure determination of the compound showed equal Ge–H bond lengths and a widening of the bridged Ge–Ge bond. This structural data and the results of density functional theory (DFT) calculations provide evidence for the first three-center two-electron Ge–H–Ge bond. The compound also represents the first example of a protonated [Ge₄]^4– cluster.

• [(μ₂-H)(η²-Ge₄)ZnPh₂]^3–, an edge-on protonated E₄ cluster establishing the first three-center two-electron Ge–H–Ge bond,
  Thomas Henneberger, Wilhelm Klein, Jasmin V. Dums, Thomas F. Fässler,
  Chem. Commun. 2018, 54, 12381–12384.
  https://doi.org/10.1039/c8cc06843g