The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR+) into the reduced [3]pseudorotaxane ([3]PR+) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR+ and [3]PR2+ are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M+) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR2+. The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR+ to 1088 nm for [3]PR2+. Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K-1 = 8.9 x 10(8) M-1 and K-2 = 3.1 x 10(6) M-1, indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR+, and [3]PR2+ shows the one-electron reductions at E-1/2 = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu+ ion. CVs of [2]PR+ show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR0 + [2]PR+ = [3]PR+ + Me(2)BPTZ(0) (K-D*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR+: Me(2)BPTZ. The driving force (Delta G(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR+ over reduced [2]PR0 by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M-1). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.