Intramolecular halogen stabilization of silylium ions directs gearing dynamics.

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(s)-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, but exchange of chlorine at silicon follows a conrotatory process.