V-Shaped Pseudo-Chains Coupled with Lone-Pair-Distorted [GeX 3 ] − Units Enable Large Birefringence in Ge(II) Hybrid Halides

Birefringent crystals are key materials for polarization and phase control, yet achieving large Δ n remains challenging. Here we report two Ge(II)-based organic–inorganic hybrid halides, (H 4 biimCl)GeCl 3 and (H 4 biimBr)GeBr 3 (H 2 biim = 2,2′-biimidazole). Single-crystal X-ray diffraction shows that protonated (H 4 biim) 2+ units form V-shaped hydrogen-bonded cationic pseudochains via N–H···X hydrogen bonds to a noncoordinating halide anion, while lone-pair-distorted trigonal-pyramidal [GeX 3 ] − units reside between the chains, creating pronounced anisotropy. The compounds exhibit large birefringence values of Δ n = 0.263 (Cl) and 0.302 (Br) at 546 nm, in good agreement with first-principles calculations (0.285 and 0.336). Halide substitution (Cl → Br) enhances the polarizability anisotropy of [GeX 3 ] − and increases Δ n . These results highlight a viable motif for birefringent crystals by coupling hydrogen-bond-directed π organization with Ge(II) lone-pair distortion.