Spectroscopic methods combined with density functional calculations are used to develop a detailed bonding description of the μ4-sulfide bridged tetranuclear Cuz cluster in N2O reductase. The ground state of Cuz has the 1CuII/3CuI configuration. The single electron hole dominantly resides on one Cu atom (CuI) and partially delocalizes onto a second Cu atom (CuII) via a CuI-S-CuII σ/σ superexchange pathway which is manifested by a CuII → CuI intervalence transfer transition in absorption. The observed excitedstate spectral features of Cuz are dominated by the S → CuI charge-transfer transitions and CuI based d-d transitions. The intensity pattern of individual S → CuI charge-transfer transitions reflects different bonding interactions of the sulfur valence orbitals with the four Cu's in the Cuz cluster, which are consistent with the individual Cu-S force constants obtained from a normal coordinate analysis of the Cuz resonance Raman frequencies and profiles. The CuI d orbital splitting pattern correlates with its distorted T-shaped ligand field geometry and accounts for the observed low gII value of Cuz in EPR. The dominantly localized electronic structure description of the Cuz site results from interactions of CuII with the two additional Cu's of the cluster (CuIII/CuIV), where the Cu-Cu electrostatic interactions lead to hole localization with no metal-metal bonding. The substrate binding edge of Cuz has a dominantly oxidized CuI and a dominantly reduced CuIV. The electronic structure description of Cuz provides a strategy to overcome the reaction barrier of N2O reduction at this CuI/CuIV edge by simultaneous two-electron transfer to N2O in a bridged binding mode. One electron can be donated directly from CuIV and the other from CuII through the CuII-S-CuI σ/σ superexchange pathway. A frontier orbital scheme provides molecular insight into the catalytic mechanism of N2O reduction by the Cuz cluster.