  We identify a class of covalent functionalizations that preserves or controls
the conductance of single-walled metallic carbon nanotubes. [2+1]
cycloadditions can induce bond cleaving between adjacent sidewall carbons,
recovering in the process the $sp^2$ 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.
