About Trihalides with TiI₃ Chain Structure: Proof of Pair Forming of Cations in β‐RuCl₃ and RuBr₃ by Temperature Dependent Single Crystal X‐ray Analyses

Single‐crystal X‐ray studies on β‐RuCl 3 and RuBr 3 at different temperatures verified, that both compounds are dimorphic and show reversible phase transitions at 206 K resp. 384 K. In the HT‐forms the Aristo‐type of the hexagonal TiI 3 ‐structure with space group P6 3 /m c m (Z = 2, β‐RuCl 3 at 293(2) K: a = 6.121(2) Å, c =5.655(2) Å, RuBr 3 at 423(3) K: 6.5215(12) Å, c= 5.8851(13) Å) has been found, in the LT‐forms the RuBr 3 ‐type structure, an orthorhombic distorted variant with space group Pmmn (Z = 4, β‐RuCl 3 at 170(3) K: a= 10.576(2) Å, b = 5.634(1) Å, c = 6.106(1) Å, RuBr 3 at 293(2) K: a = 11.2561(16) Å, b = 5.8725(12) Å, c = 6.4987(9) Å). A hexagonal closest packing of X ‐ anions forms the basis of an arrangement of infinite chains with face‐connected [RuX 6/2 ] octahedra. While in the chains of the hexagonal HT‐forms the Ru‐Ru‐distances are identical (d(Ru‐Ru) = 2.8275(10) Å for β‐RuCl 3 , d(Ru‐Ru) = 2.9425(6)Å for RuBr 3 ), in the orthorhombic structures the chains are distorted through pairing of the ruthenium(III) atoms (d(Ru‐Ru) = 2.6328(14) Å / 3.0010(15) Å for β‐RuCl 3 at 170(3) K, d(Ru‐Ru) = 2.765(1) Å / 3.108(1) Å for RuBr 3 at 293(2) K). The hexagonal metric with a/c= √3 holds also for the orthorhombic LT‐forms. Large crystals and the final products of the phase transition from HT‐ to LT‐forms are pseudomerohedral twins of three twin domains with nearly equal amounts complicating proof and analysis of the LT‐forms.