The relative solvent-separated ion pair (SSIP) lithium acidity (~ KUITHF) and contact ion pair (CIP) cesium acidity (~ KC~ ITHF) were obtained for 9-fluorenyltrimethylsilane (1)(21.3, 21.6, respectively) and 9-fluorenyl-tert-butyldimethylsilane (2)(20.3, 20.6, respectively) in THF. Values for PKC~ ITHF were determined at 25" C for@-biphenylylmethy1)-tert- butyldimethylsilane (3), 35.4, benzyltrimethylsilane (4), 37.5, a, a-bis (trimethylsily1) ...