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Cameron TS, Decken A, Dionne I, Fang M, Krossing I, Passmore J: Approaching the gas-phase structures of [AgS8]+ and [AgS16]+ in the solid state. Chemistry. 2002 Aug 2;8(15):3386-401.
Upon treating elemental sulfur with [AgSbF (6)], [AgAl (hfip)(4)], [AgAl (pftb)(4)] (hfip=OCH (CF (3))(2), pftb =OC (CF (3))(3)) the compounds [Ag (S (8))(2)][SbF (6)] (1), [AgS (8)][Al (hfip)(4)] (2), and [Ag (S (8))(2)](+)[[Al (pftb)(4)](-) (3) formed in SO (2) (1), CS (2) (2), or CH (2) Cl (2) (3). Compounds 1-3 were characterized by single-crystal X-ray structure determinations: 1 by Raman spectroscopy, 2 and 3 by solution NMR spectroscopy and elemental analyses. Single crystals of [Ag (S (8))(2)](+)[Sb (OTeF (5))(6)](-) 4 were obtained from a disproportionation reaction and only characterized by X-ray crystal structure analysis. The Ag (+) ion in 1 coordinates two monodentate SbF (6) (-) anions and two bidentate S (8) rings in the 1,3-position. Compound 2 contains an almost C (4v)-symmetric [AgS (8)](+) moiety; this is the first example of an eta (4)-coordinated S (8) ring (d (Agbond;S)=2.84-3.00 A). Compounds 3 and 4, with the least basic anions, contain undistorted, approximately centrosymmetric Ag (eta (4)-S (8))(2) (+) cations with less symmetric eta (4)-coordinated S (8) rings (d (Agbond;S)=2.68-3.35 A). The thermochemical radius and volume of the undistorted Ag (S (8))(2) (+) cation was deduced as r (therm)(Ag (S (8))(2) (+))=3.378+ 0.076/-0.120 A and V (therm)(Ag (S (8))(2) (+))=417+4/-6 A (3). AgS (8) (+) and several isomers of the Ag (S (8))(2) (+) cation were optimized at the BP86, B3LYP, and MP2 levels by using the SVP and TZVPP basis sets. An analysis of the calculated geometries showed the MP2/TZVPP level to give geometries closest to the experimental data. Neither BP86 nor B3LYP reproduced the longer weak dispersive Agbond;S interactions in Ag (eta (4)-S (8))(2) (+) but led to Ag (eta (3)-S (8))(2) (+) geometries. With the most accurate MP2/TZVPP level, the enthalpies of formation of the gaseous [AgS (8)](+) and [Ag (S (8))(2)](+) cations were established as Delta (f) H (298)([Ag (S (8))(2)](+), g)=856 kJ mol (-1) and Delta (f) H (298)([AgS (8)](+), g)=902 kJ mol (-1). It is shown that the [AgS (8)](+) moiety in 2 and the [AgS (8)](2) (+) cations in 3 and 4 are the best approximation of these ions, which were earlier observed by MS methods. Both cations reside in shallow potential-energy wells where larger structural changes only lead to small increases in the overall energy. It is shown that the covalent Agbond;S bonding contributions in both cations may be described by two components: i) the interaction of the spherical empty Ag 5s (0) acceptor orbital with the filled S 3p (2) lone-pair donor orbitals and ii) the interaction of the empty Ag 5p (0) acceptor orbitals with the filled S 3p (2) lone-pair donor orbitals. This latter contribution is responsible for the observed low symmetry of the centrosymmetric Ag (eta (4)-S (8))(2) (+) cation. The positive charge transferred from the Ag (+) ion in 1-4 to the coordinated sulfur atoms is delocalized over all the atoms in the S (8) ring by multiple 3p (2)--> 3sigma* interactions that result in a small long-short-long-short Sbond;S bond-length alternation starting from S1 with the shortest Agbond;S length. The driving force for all these weak bonding interactions is positive charge delocalization from the formally fully localized charge of the Ag (+) ion. |
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