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# 2000 International Union of Crystallography Printed in Great Britain ± all rights reserved Single-crystal X-ray structures are presented for three forms of 5H,10H-dithiolo[2,3-b]-2,5-benzodithiocine-2-thione. The (at 150 K) and 0 (at ambient) forms are very similar and differ only in the presence of crystallographic m symmetry in the molecules of 0, which is absent in the case of . This pair is related by an order±disorder transition. The phase (also determined at 150 K) has a different structure in terms of the molecular packing from either of the other two and therefore constitutes a true polymorph. Molecular mechanics calculations indicated that the most stable CHCl3-solvated conformations for the title compound were a pair of twisted U-shaped enantiomers, UR and UL, i.e. similar to the arrangements found in the and phases, with the low-lying saddle point between them corresponding to the situation in the 0 phase. These calculations also indicated that the most stable CHCl3solvated conformation for the related dibromo-5H,10Hdithiolo[2,3-b]-2,5-benzodithiocine-2-thione was Z-shaped, in agreement with the crystal structure determined earlier for its DMSO solvate [Wang et al. (1998). Synthesis, pp. 1615±1618]. Received 24 February 2000 Accepted 28 July 2000

Crystallographic and molecular mechanics investi- gation of an order±disorder transition and dimorphism in 5H,10H-dithiolo[2,3-b]-2,5-benzo- dithiocine-2-thione