This study details the electronic structure of the heme peroxo copper adduct {[(F-8)Fe(DCHIm)]center dot O-2 center dot[Cu(AN)]}(+) (LSAN) in which O-2(2-) bridges the metals in a mu-1,2 or "end-on" configuration. LSAN is generated by addition of coordinating base to the parent complex {[(F-8)Fe]-O-2-[Cu(AN)]}(+) (HSAN) in which the O-2(2-) bridges the metals in an mu-eta(2):eta(2) or "side-on" mode. In addition to the structural change of the O-2(2-) bridging geometry, coordination of the base changes the spin state of the heme fragment (from S = 5/2 in HSAN to S = 1/2 in LSAN) that results in an antiferromagnetically coupled diamagnetic ground state in LSAN. The strong ligand field of the porphyrin modulates the high-spin to low-spin effect on Fe-peroxo bonding relative to nonheme complexes, which is important in the O-O bond cleavage process. On the basis of DFT calculations, the pound state of LSAN is dependent on the Fe-O-O-Cu dihedral angle, wherein acute angles (