Approaching the gas-phase structures of [AgS8]+ and [AgS16]+ in the solid state

Upon treating elemental S with [AgSbF6], [AgAl(hfip)4], [AgAl(pftb)4] (hfip = OCH(CF3)2, pftb = OC(CF3)3), [Ag(S8)2][SbF6] (1), [AgS8][Al(hfip)4] (2), and [Ag(S8)2]+[Al(pftb)4]- (3) formed in SO2 (1), CS2 (2), or CH2Cl2 (3). 1-3 Were characterized by single-crystal x-ray structure detns.: 1 by Raman spectroscopy, 2 and 3 by soln. NMR spectroscopy and elemental analyses. Single crystals of [Ag(S8)2]+[Sb(OTeF5)6]- (4) were obtained from a disproportionation reaction and only characterized by x-ray crystal structure anal. The Ag+ ion in 1 coordinates two monodentate SbF6- anions and two bidentate S8 rings in the 1,3-position. 2 Contains an almost C4v-sym. {AgS8}+ moiety; this is the 1st example of an h4-coordinated S8 ring (d(Ag-S) = 2.84-3.00 .ANG.). 3 And 4, with the least basic anions, contain undistorted, approx. centrosym. Ag(h4-S8)2+ cations with less sym. h4-coordinated S8 rings (d(Ag-S) = 2.68-3.35 .ANG.). The thermochem. radius and vol. of the undistorted Ag(S8)2+ cation was deduced as rtherm(Ag(S8)2+) = 3.378+0.076/-0.120 .ANG. and Vtherm(Ag(S8)2+) = 417+4/-6 .ANG.3. AgS8+ and several isomers of the Ag(S8)2+ cation were optimized at the BP86, B3LYP, and MP2 levels by using the SVP and TZVPP basis sets. An anal. of the calcd. geometries showed the MP2/TZVPP level to give geometries closest to the exptl. data. Neither BP86 nor B3LYP reproduced the longer weak dispersive Ag-S interactions in Ag(h4-S8)2+ but led to Ag(h3-S8)2+ geometries. With the most accurate MP2/TZVPP level, the enthalpies of formation of the gaseous [AgS8]+ and [Ag(S8)2]+ cations were established as DfH298([Ag(S8)2]+, g) = 856 kJ mol-1 and DfH298([AgS8]+, g) = 902 kJ mol-1. The {AgS8}+ moiety in 2 and the {AgS8}2+ cations in 3 and 4 are the best approxn. of these ions, which were earlier obsd. by MS methods. Both cations residue in shallow potential-energy wells where larger structural changes only lead to small increases in the overall energy. The covalent Ag-S bonding contributions in both cations may be described by two components: (i) the interaction of the spherical empty Ag 5s0 acceptor orbital with the filled S 3p2 lone-pair donor orbitals and (ii) the interaction of the empty Ag 5p0 acceptor orbitals with the filled S 3p2 lone-pair donor orbitals. This latter contribution is responsible for the obsd. low symmetry of the centrosym. Ag(h4-S8)2+ cation. The pos. charge transferred from the Ag+ ion in 1-4 to the coordinated S atoms is delocalized over all the atoms in the S8 ring by multiple 3p2 -> 3s* interactions that result in a small long-short-long-short S-S bond-length alternation starting from S1 with the shortest Ag-S length. The driving force for all these weak bonding interactions is pos. charge delocalization from the formally fully localized charge of the Ag+ ion. [on SciFinder (R)]