A systematic investigation has been made into the degree of silylium ion character in species of the type i-Pr 3 SiY where Yis a weakly coordinating hexahalo-carborane anion, X 6 -CB 11 H 6 -, with X ) Cl, Br, I. By X-ray crystal structural criteria such as the average C-Si-C angle (117.3(4)°, 117.0(13)°, and 115.6(9)°for X ) Cl, Br, I, respectively) and the downfield 29 Si chemical shift (115, 110, and 97 ppm for X ) Cl, Br, I, respectively) the hexachloro-carborane anion is the least coordinating. Thus, the "ion-like" species i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δbecomes the closest structural approach to date to a condensed phase silylium ion. With the aid of ab initio molecular orbital theory and silicon chemical shift calculations we address the question of how much silylium ion character this species represents. From structural, energetic, and NMR criteria (where expectation values for the condensed phase 29 Si chemical shifts are revised downward) we conclude that i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δmay have >50% silylium ion character.
Crystal structures have been determined for three Ag(CB 11 H 6 X 6 ) compounds (X = Cl, Br or I) crystallized from arene solvents. They are all one-dimensional co-ordination polymers in the solid state with hexahalogenocarborane anions acting as bridging ligands. The co-ordination geometries of these three salts are, however, unique. The type and number of donors to silver() varies from one to another and is not particularly predictable.Icosahedral carboranes constitute a new class of very weakly co-ordinating anions. 1,2 The 7-12 hexahalogenated derivatives CB 11 H 6 X 6 Ϫ (X = Cl, Br or I), are larger, more thermodynamically stable, more chemically inert, less co-ordinating and less nucleophilic than the parent CB 11 H 12 Ϫ ion. 3 They play a very important role in stabilizing co-ordinatively unsaturated cations such as the silylium ion (R 3 Si ϩ ) 4-7 and the four-co-ordinate 5,10,15,20-tetraphenylporphyrinatoiron() ion, [Fe(tpp)] ϩ . 8 Silver() salts of carborane anions are very useful halide metathesis reagents for introducing weakly co-ordinating anions into complexes and they are finding immediate applications in metathesis, catalysis and oxidation chemistry. 2-12 They are also of structural interest in their own right because they illustrate the subtlety and diversity of co-ordination chemistry when anions have comparable donor ability to solvent molecules. Examples of crystallographically characterized silver() carborane complexes include
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