Approaching the Gas‐Phase Structures of [AgS 8 ] + and [AgS 16 ] + in the Solid State

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 η 4 ‐coordinated S 8 ring ( d (AgS)=2.84–3.00 Å). Compounds 3 and 4 , with the least basic anions, contain undistorted, approximately centrosymmetric Ag( η 4 ‐S 8 ) 2 + cations with less symmetric η 4 ‐coordinated S 8 rings ( d (AgS)=2.68–3.35 Å). 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 Å and V therm (Ag(S 8 ) 2 + )=417+4/−6 Å 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 AgS interactions in Ag( η 4 ‐S 8 ) 2 + but led to Ag( η 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 Δ f H 298 ([Ag(S 8 ) 2 ] + , g)=856 kJ mol −1 and Δ 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 AgS 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 observedlow symmetry of the centrosymmetric Ag( η 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 →3σ* interactions that result in a small long‐short‐long‐short SS bond‐length alternation starting from S1 with the shortest AgS length. The driving force for all these weak bonding interactions is positive charge delocalization from the formally fully localized charge of the Ag + ion.