THE growing interest in miniaturization of electronic components is stimulating research on molecular assemblies with device-like functionalities(1-5). Molecule-based devices have been reported that might act as sensors(6-8), diodes(9-11), logic gates(12) and switches(5,13-19) Molecular switches should ideally be able to respond controllably and reversibly to external triggers(7,12,14,15,20,21). Here we report the synthesis of molecular redox switches based on helical metal complexes(22-29) in which an iron ion can occupy one of two distinct binding cavities. Reversible translocation of the metal ion between these sites is achieved by chemical oxidation and reduction, owing to the different coordination preferences of the Fe(II) and Fe(III) states, and can be readily monitored spectroscopically.