The structural characteristics and reactivity of the gas-phase FSi(OCH3)(2)(OCH2)(-) ion were investigated by a combination of ab initio calculations and FT-ICR techniques. The theoretical calculations for different possible structures reveal that carbanion and alkoxide ion type structures lead to ring closure upon geometry optimization to yield two different cyclic fluoride-siloxirane structures. The FSi(OCH3)(2)(cyc-OCH2)(-) ions containing the elusive siloxirane ring are estimated to be extremely stable with respect to F-(69 kcal mol(-1)) dissociation in agreement with earlier calculations on simpler systems. Experimentally, this ion is formed as a minor product (7%) in the gas-phase ion/molecule reaction of F- with Si(OMe)(4) and is observed to undergo readily fluoride transfer to the parent neutral. This strongly suggests an ion with a structure corresponding to a fluoride adduct of a siloxirane species, Reaction of FSi(OCH3)(2)(OCH2)(-) with BF3, hexafluorobenzene, and gas-phase acids more acidic than ethanol further suggests that this ion is capable of reacting as an alkoxide type nucleophile or base. This latter behavior has been associated with the possibility of ring opening of the siloxirane in the collision complex that mediates this ion/molecule reaction.