Metalloproteins utilize bimetallic cores containing bridging oxido/hydroxido units to facilitate chemical transformations that are important in energy conversion, metabolism, and synthesis of biomolecules. Although the role of bridging ligand in regulating the spin exchange coupling between metal centers is well-established, its effect on the electronic structure of metal ion(s), including spin state transitions, is underexplored. The research described herein investigates this correlation by examining the molecular structures and spectroscopic properties of homo- and heterobimetallic complexes with the general formulation [(TMTACN)M–(μ-O(H))–M’poat]z+ where M = FeIII, CoIII; M’ = FeIII, MnIII, GaIII; z = 1, 2; TMTACN = 1,4,7-trimethyl-1,4,7-triazacyclononane; and [poat]3– = N,N′,N″-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido)). The metal ions within the M–(μ-O(H))–M’ core are systematically varied to examine the resulting changes in thermodynamic parameters and the acidity of the proton on the bridging-hydroxido unit. Additionally, the paramagnetism of the [CoIII–(μ-O(H))–FeIII] series can be controlled through reversible protonation of the bridging unit, where the hexacoordinate CoIII switches between SCo = 0 and 2 states while the spin on the FeIII site remains constant. The findings from this study provide valuable insights into the correlation between proton movement and spin state changes, which can be leveraged to enhance the catalytic activity of metal catalysts and tune the electronic properties of magnetic materials.