Introduetlon. The crystal structures of the title compounds were determined as part of a series of investigations, comprising both solid and solution chemistry, on coordination compounds with potentially tridentate ligands containing a thioether or disulphide group. Colourless thin plates of 3,3'-thiodipropionic acid and 3,3'-dithiodipropionic acid were grown from a warm solution (aqueous for TDPA and ethanolic for DTDPA) which was left to evaporate at room temperature. Crystals were not air sensitive and precession photographs showed those of TDPA to be orthorhombic, with systematic absences: hk0: h + k = 2n + 1; hOl: h = 2n + 1; Okl: l = 2n + I, corresponding to space group Pcan [non-standard setting _+ (x,y,z; ½ -x, ½-y, ½-z; ½--x, ½ + y, z; x, V, ½ + z)], and those of DTDPA to be monoclinic, with systematic absences:hkl: h + k = 2n + 1; hOl: l = 2n + 1, consistent with space groups C2/c and Cc (the structure was refined successfully in C2/c).Single crystals, 0.60 x 0.30 x 0.15 mm for TDPA and 0.75 x 0.50 x 0.15 mm for DTDPA, were used for intensity-data collection on a Nonius CAD-4F automated diffractometer using graphite-monochromated Mo Ka radiation. Accurate unit-cell dimensions and the orientation matrix were obtained by least 0567-7408/82/010338-03501.00 squares from the setting angles of 25 reflexions. The intensities of reflexions with 0 < 25 o were measured by an o)/20 scan, a variable scan rate and an o~ scan angle of (1.5 + 0.35 tan 0) ° for TDPA and (1.3 + 0.35 × tan 0) ° for DTDPA. Some low-0 reflexions of DTDPA were so intense that they had to be recollected with one fifth of the usual filament current of the X-ray generator to avoid dead-time losses. 168 reflexions for TDPA and 174 for DTDPA with I < 3o(1), where o(I) is the standard deviation based on simple counting statistics, were not included in the subsequent calculations, which were based on the remaining 523 and 626 independent reflexions for TDPA and DTDPA, respectively. Lorentz and polarization corrections were applied but no correction was made for absorption.The structures were solved by the heavy-atom method and Fourier techniques, and refined using full-matrix least-squares methods. The S atom in TDPA is located in the special position 2(c). After refinement of positional and anisotropic thermal parameters for all non-hydrogen atoms, difference maps clearly indicated H-atom positions, which were included with isotropic temperature factors in the next cycles, C-H bonds being constrained to have the same length and angles as in sp 3 hybridization. In the last stage of refinement, each reflexion was assigned a weight w = 1/,,,r=X' n ~ ,4rTr(X) where n is the number of coefficients ,4 r for a Chebyshev series, T r is the polynomial function and X = IFol/bFo(max.)l. Three coefficients ,4 r were used, with values 101.1, 136.9 and 39.9 for TDPA and 15.1, 18.1 and 5.2 for DTDPA (Rollett, 1965; Carruthers & Watkin, 1979). The use of the weighting-scheme function shortened hydrogenbond lengths by 0.04-0.06 A. Final difference maps had no pe...

The crystal and molecular structures of the title compounds have been determined by X-ray methods. is best regarded as a planar group in all four compounds. Cohesion of the crystal structures is effected by hydrogen-bonding networks in (I) and (II) and van der Waals packing forces in (III) and (IV).