2,6-Bis(2,6-difluorophenyl)phenyldimethylsilanium ion (1a) adopts a ground-state C-2 trigonal-bipyramidal geometry in which fluorines from opposing 2,6-difluorophenyl groups coordinate to the apical positions of the pentavalent silyl cation. Exchange of fluorine at silicon occurs by a disrotatory gearing mechanism wherein one fluorine remains coordinated to silicon throughout the circuit. The "cogwheel" transition state is C-2-symmetric, with one ring having a dihedral angle of 0 degrees and the other a dihedral angle of 90 degrees. This correlated dynamic process is a function of the coordinating halogen. The chloro derivative (1b) adopts a similar ground-state geometry,