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In the 21 reported crystal structures of symmetric disulfides whose sulfur atoms are bound to sp2 carbon atoms, an empirical relationship is found between the S-S bond length and the X-C-S-S, X = C or N, torsion angle. When X-C-S-S is within about 20°of 90°, the average S-S bond length is 2.08 Á (2.059-2.108 Á) in six structures; when X-C-S-S is within about 20°of 0°, S-S averages to 2.03 Á (1.999-2.050 Á) in 15 structures. The C-S-S-C torsion angles are generally within about 20°of 90°in those molecules. In addition, it is noted that the less frequently encountered X-C-S-S angles near 90°o ccur only when one strongly electron-donating substituent is present in a position ß to the disulfide group in each of the R groups of the RSSR molecule, or when both ß positions on each R group are substituted, independent of electronegativity. (If an aromatic ring of an R group is bonded to the disulfide group, the nomenclature ortho instead of ß is usually used.) A relatively neutral or electron-withdrawing group in only one ß position of each R group (or any substituents at any more distant positions) allows the X-C-S-S torsion angles to be near 0°, and the S-S bond, therefore, to be short. This result is supported by the crystal and molecular structure of 3,3'-dihydroxydi-2-pyridyl disulfide, C10H8N2S2O2, which has been determined by single-crystal X-ray diffraction techniques using counter methods, and has been refined by full-matrix least-squares procedures to a final conventional R index of 0.035. The yellow monoclinic crystals form in the space group P2\jc with a = 7.165 (1) Á, b = 7.656 (1) Á, c = 19.517 (3) Á, /3 = 96.77 (1)°, and four molecules per unit cell. The average N-C-S-S torsion angle (9°) indicates that the S-S bond (2.0180 (7) Á) is nearly in the plane of each pyridine ring. The C-S-S-C torsion angle is 93.2°, and the average S-C bond length is 1.782 Á. The empirical rules presented also successfully describe the 11 transition metal ion complexes involving these disulfides whose structures are known. The C-S-S-C torsion angle has a role in governing the S-S bond length, but is important only when X-C-S-S is within about 20°of 90°or when C-S-S-C deviates by about 40°o r more from 90°.

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