The multiheme cytochrome c involves extensive interaction among the heme centers to enable them to perform a wide variety of enzymatic activities. In an attempt to exploit such heme–heme interactions in the synthetic diheme, an ethane-bridged diiron(III) porphyrin dimer has been utilized that can switch easilybetween syn and anti conformations due to highly flexible nature of the bridge. Upon protonation using 5% aqueous Brønsted acid, the dichloromethane solution of diiron(III)-µ-oxo porphyrin dimer immediately changes its color from green to red leading to the formation of a series of µ-hydroxo complexes. However, long exposure of the Brønsted acid converts the µ-hydroxo complex to five-coordinate diiron(III) porphyrin dimer in which the counter anion switches its position to act as an axial ligand. In the present study, the investigation has been extended further using Brønsted acids containing strongly coordinating anions such as HN₃, HNCS and l-(+)-Lactic acid (HLA) in which the anions eventually coordinate to iron centers directly as axial ligands. A comprehensive account of the anion-mediated spin state change in the ethane-bridged diiron(III) porphyrin dimer has also been presented here.