Stable Stannylium Cations in Condensed Phases

Department of Chemistry, Northwestern University, Evanston, Illinois, USA 

Tricoordinate tin, until recently, has been unknown in condensed phases, due to its high electrophilicity. Thermodynamic stability in the gas phase testifies to the kinetic roots of its instability, particularly in the liquid phase. The ideal tricoordinate geometry is planar and trigonal, as viewed in 1 from above or below, and in 2 from the side on. 

It is expected that each R-Sn-R angle is very close to 120°, so that they must sum to 360°. The empty 4p orbital should have identical, large lobes above and below the plane defined by the tin atom and its three 

Much research has been carried out in many contexts to create anions with low nucleophilicity. In the specific context of low valent silyl cations, the anion families that have proved most successful have been perfluoroaromatics introduced by Lambert and Zhang and carboranes introduced by Reed and coworkers, reported in adjacent publications in 1993. Prior to use of these anions, unsuccessful investigations were reported with anions such as perchlorate, tetrafluoroborate, tetraphenylborate, and hexachloroantimonate. All successful preparations of stannylium cations have used perfluoroaromatic anions, particularly tetrakis(pentafluorophenyl)borate (TPFPB). 

Coordination with solvent or anion can generate several different geometries. When a stannylium ion coordinates with a neutral solvent molecule S, either a tetracoordinate species (3, referred to as a solvated stannyl cation) or a pentacoordinate species (4) can form. Although these materials qualify as stannyl 

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