N-3(-) binding to a functional model of CcO is investigated in its Fe3+, Fe3+Cu+, and Fe3+Cu2+ forms. A combination of EPR and FTIR indicates that N-3(-) binds in a bridging mode in the bimetallic sites and signature N-3(-) bands are identified for several forms of N-3(-) binding to the site. The presence of the distal metal increases the binding affinity of N-3(-). This bridging enables antiferromagnetic interaction between the two metal centers in the Fe3+Cu2+ state, which results in an EPR-silent ground state.