Four Crystalline Forms of (Tetrahydrothiophene)trichloridogold(III): Polymorphism and Reversible Low‐temperature Phase Transitions

The title compound (tht)AuCl3, previously known to crystallize at –95 °C in space group C2 with Z = 4 (form I), converts reversibly to form II (P21/n, Z = 8) at ca. –150 °C; the b axis doubles and the centering is lost. A few crystals of a rarer form III (Pna21, Z = 4) were also investigated; these transform reversibly to form IV (P21/c, Z = 4) at ca. –160 °C. In all forms, stacks of molecules are formed, with short Au···Cl contacts within the stacks. For forms I and II, adjacent stacks are connected by C–H···Cl interactions; more striking for forms III/IV are short Cl···Cl contacts between the stacks. Detailed physico‐chemical studies of the relationships between the various phases proved to be impossible, largely because form III could not be obtained reproducibly in significant quantities.