Structure and bonding of diiodine adducts of the sulfur-rich donors 1,3-dithiacyclohexane-2-thione (ptc) and 4,5-ethylenedithio-1,3-dithiole-2-thione(ttb)

The reactions of I-2 with ptc and ttb (title ligands) have been investigate d in CHCl3 solution at different temperatures by spectrophotometry. A least -squares method procedure provided evidence for the formation of the 1:1 ad ducts. Crystals of the latter have been analyzed by X-ray diffraction metho ds (both monoclinic, P2(1)/c; ptc.I-2, a = 8.691(6) Angstrom, b = 9.010(6) Angstrom, c = 13.237(5) Angstrom, beta = 103.43(2)degrees, Z = 4, R = 0.030 5; ttb.I-2, a = 12.090(6) Angstrom, b = 6.433(5) Angstrom, c = 15.731(6) An gstrom, beta = 99.30(2)degrees, Z = 4, R = 0.0419). Both structures show th at the thionic sulfur tin any case a CS3 group inserted in a ring) is bound almost collinearly with the diiodine molecule. The IZ(S-I) separations are 2.755(2) and 2.805(3) Angstrom in the ptc.I-2 and ttb.I-2 adducts, respect ively, while d(I-I) is practically the same (2.812(2) Angstrom). An evident stereochemical difference is that the S-I-I moiety is nearly coplanar with the CS3 group in ptc.I-2 while it is upright in ttb.I-2; However, the feat ure is not expected to cause a major electronic difference. In order to rep roduce the structural features, different ab initio approaches have been at tempted, with the best results being obtained with the density functional m ethod (DPT). Despite the S-l distances which are slightly longer than the e xperimental ones (by ca. 0.25 Angstrom), the distribution of filled and emp ty frontier molecular orbitals (MOs) allows a good interpretation of the vi sible spectra. Also a rationalization of the sigma electronic density distr ibuted over the three centers S-I-I has been attempted by qualitative MO th eory (EHMO method). Provided the good agreement with the higher level calcu lations, the perturbation theory arguments highlight the variable sp hybrid ization at the central iodine atom as the electronic factor of importance. The strength of the donor (D) affects significantly the redistribution of s ix electrons over four atomic orbitals, and the classic model is revised as a four-orbital/six-electron one. Thus, it is pointed out that a major four -electron repulsion is exerted over the D-I or the I-I linkages with major consequences for their respective lengths.