Interest in multimetal metalloproteins frequently results from the observation of spin coupling phenomena because they give clues to the presence of proximate paramagnetic centers. It is not usually the spin coupling per se that is important to the biochemistry of a metalloprotein but an understanding of the nature of the spin interaction is inextricably associated with the deduction of the structure of the metal sites. And it seems that the more completely the normal magnetic signatures of a metal are erased by spin coupling, the more challenging becomes the elucidating of structure. The diamagnetism of copper(II) hemocyanin derivatives and the EPR silent heme a 3–copper site of cytochrome oxidase are cases in point. The interralationship of spin coupling and structure has dictated that progress in understanding magnetic interactions has been dependent on a detailed knowledge of structure, usually gained from X-ray crystallography. So it is, with the synthetic model compound approach to metalloproteins, that the design and synthesis of complexes which are amenable to definitive structural characterization is critically important. In this paper, the design, synthesis and structural characterization of spin coupled models for hemocyanin and cytochrome oxidase are discussed within the context of using magnetic coupling phenomena to raw structural implications for their binuclear active sites. We have recently reported [1] an azide/alkoxide dibridged binuclear copper(II) complex which, like hemocyanin, is diamagnetic (Fig. 1, X = N 3). The rationale for understanding this strong antiferromagnetic coupling is based on a superexchange pathway via the bridging for orbital overlap of the (d x 2 − y 2 ) 1 ground states of the approximately ▪tetragonal copper centers. In contrast, the trigonal bipyramidal copper centers in the corresponding acetate complex (Fig. 1, X = OAc) are essentially spin free (μ = 1.83 per Cu). These complexes endorse the proposal that diamagnetism in oxy- and methemocyanin (and Type III copper) is the result of dibridged tetragonal copper(II) centers with at least one O-bridged ligand such as alkoxide. The mostpopular explanation for the absence of an EPR signal for the active site of resting state cytochrome oxidase is spin coupling between high-spin ferric heme a 3 and a proximate copper(II) center [2]. Figure 2 illustrates a ligand whose complexes with iron(III) in the porphyrin base and copper(II) coordinated in the bisbenzimidazole superstructure yield isolable crystalline materials of analytical and spectroscopic purity. Their magnetic properties are under investigation.