During bacterial denitrification, two-electron reduction of N2O occurs at a [Cu-4(mu(4)-S)] catalytic site (Cu-Z*) embedded within the nitrous oxide reductase (N2OR) enzyme. In this Communication, an amidinate-supported [Cu-4(mu(4)-S)] model cluster in its one-hole (S = 1/2) redox state is thoroughly characterized. Along with its two-hole redox partner and fully reduced clusters reported previously, the new species completes the two-electron redox series of [Cu-4(mu(4)-S)] model complexes with catalytically relevant oxidation states for the first time. More importantly, N2O is reduced by the one-hole cluster to produce N-2 and the two-hole cluster, thereby completing a closed cycle for N2O reduction. Not only is the title complex thus the best structural model for Cu-Z* to date, but it also serves as a functional Cu-Z* mimic.