The coordination of low-valent group 13 metals, i.e., Al(I), Ga(I), In(I), or Tl(I) with transition metals can lead to interesting complexes. These cations can, for example, serve as “naked” acceptor ligands in electron-rich transition-metal compounds or as a bridging ligand between transition-metal centers to form linear complexes. However, examples of this type of linear complex are limited.

Zhong-Ming Sun, Nankai University, Tianjin, China, and colleagues have synthesized a compound that contains a linear Nb–Ga(I)–Nb fragment, i.e., K(2.2.2-crypt)]₃[Ga@Bi₁₀(NbMes)₂] (partial structure pictured, Mes = mesitylene). The team prepared the compound via a reaction of K₅Ga₂Bi₄ with NbMes₂ in the presence of 2.2.2-crypt in ethylenediamine. The product was obtained in the form of dark green crystals in a yield of 21 %.

Single-crystal X-ray diffraction showed that the anionic cluster [Ga@Bi₁₀(NbMes)₂]^3– features an endohedral Ga atom in an Nb₂Bi₁₀ cage with Mes groups at the Nb atoms. The team studied the bonding situation in the cluster using quantum-chemical calculations and found that the Nb–Ga and Nb–Bi bonds are covalent in nature, with the Ga⁺ ion forming electron-sharing bonds with Nb via sp-hybrid orbitals.

According to the researchers, [Ga@Bi₁₀(NbMes)₂]^3– is the first single Ga(I)-containing metal cluster, as well as the first compound with Nb–Ga and Nb–Bi bonds.

• [Ga@Bi₁₀(NbMes)₂]³⁻: a linear Nb–Ga^I–Nb filament coordinated by a bismuth cage,
  Lei Qiao, Tao Yang, Gernot Frenking, Zhong-Ming Sun,
  Chem. Commun. 2023.
  https://doi.org/10.1039/d3cc00631j