We identify a class of covalent functionalizations that preserve or control the conductance of single-walled metallic carbon nanotubes. $[2+1]$ cycloadditions can induce bond cleaving between adjacent sidewall carbons, recovering in the process the $s{p}^2$ hybridization and the ideal conductance of the pristine tubes. This is radically at variance with the damage permanently induced by other common ligands, where a single covalent bond is formed with a sidewall carbon. Chirality, curvature, and chemistry determine bond cleaving, and in turn the electrical transport properties of a functionalized tube. A well-defined range of diameters can be found for which certain addends exhibit a bistable state, where the opening or closing of the sidewall bond, accompanied by a switch in the conductance, could be directed with chemical, optical, or thermal means.

• Received 6 April 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.116801