title: 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).
abstract: The reactions of I(2) with ptc and ttb (title ligands) have been investigated in CHCl(3) solution at different temperatures by spectrophotometry. A least-squares method procedure provided evidence for the formation of the 1:1 adducts. Crystals of the latter have been analyzed by X-ray diffraction methods (both monoclinic, P2(1)/c; ptc.I(2), a = 8.691(6) Å, b = 9.010(6) Å, c = 13.237(5) Å, beta = 103.43(2) degrees, Z = 4, R = 0.0305; ttb.I(2), a = 12.090(6) Å, b = 6.433(5) Å, c = 15.731(6) Å, beta = 99.30(2) degrees, Z = 4, R = 0.0419). Both structures show that the thionic sulfur (in any case a CS(3) group inserted in a ring) is bound almost collinearly with the diiodine molecule. The d(S-I) separations are 2.755(2) and 2.805(3) Å in the ptc.I(2) and ttb.I(2) adducts, respectively, while d(I-I) is practically the same (2.812(2) Å). An evident stereochemical difference is that the S-I-I moiety is nearly coplanar with the CS(3) group in ptc.I(2) while it is upright in ttb.I(2). However, the feature is not expected to cause a major electronic difference. In order to reproduce the structural features, different ab initio approaches have been attempted, with the best results being obtained with the density functional method (DFT). Despite the S-I distances which are slightly longer than the experimental ones (by ca. 0.25 Å), the distribution of filled and empty frontier molecular orbitals (MOs) allows a good interpretation of the visible spectra. Also a rationalization of the sigma electronic density distributed over the three centers S-I-I has been attempted by qualitative MO theory (EHMO method). Provided the good agreement with the higher level calculations, the perturbation theory arguments highlight the variable sp hybridization at the central iodine atom as the electronic factor of importance. The strength of the donor (D) affects significantly the redistribution of six 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.
authors.affiliation: Dipartimento di Chimica Generale ed Inorganica, Centro di Studio per la Strutturistica Diffrattometrica del CNR, Università degli Studi di Parma, Viale delle Scienze 78, I-43100 Parma, Italy, Dipartimento di Chimica e Tecnologie Inorganiche e Metallorganiche, Università degli Studi di Cagliari, Cittadella di Monserrato, I09131 Monserrato-Cagliari, Italy, Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione (ISSECC, CNR), via Nardi 39, I-50132 Firenze, Italy, and Dipartimento di Scienze Chimiche, Università degli Studi di Cagliari, Cittadella di Monserrato, I09131 Monserrato-Cagliari, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Bigoli
authors.forename: Paola
authors.initials: P
authors.lastname: Deplano
authors.forename: Andrea
authors.initials: A
authors.lastname: Ienco
authors.forename: Carlo
authors.initials: C
authors.lastname: Mealli
authors.forename: Maria Laura
authors.initials: ML
authors.lastname: Mercuri
authors.forename: Maria Angela
authors.initials: MA
authors.lastname: Pellinghelli
authors.forename: Gloria
authors.initials: G
authors.lastname: Pintus
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Saba
authors.forename: Emanuele F.
authors.initials: EF
authors.lastname: Trogu