Using a combination of electronic spectroscopies, electronic structural descriptions have been developed for a series of binuclear CuA-type centers in Bacillus subtilis CcO and engineered into the blue copper proteins Pseudomonas aeruginosa azurin and Thiobacillus versutus amicyanin. Parallel descriptions are developed for two structurally characterized mixed-valence (MV) and homovalent (II,II) synthetic copper thiolate dimers. Assignment of the excited-state spectral features allows the electronic structures of CuA and the MV model to be understood and compared in relation to their copper coordination environments. These electronic structural descriptions are supported by SCF-Xα-SW MO calculations, which are used to test systematically the effects of major structural perturbations linking the MV model geometry to that of CuA. It is determined that both Cu−Cu compression and removal of the axial ligands are critical determinants of the orbital ground state in these dimers. The weakened axial interactions ...