Spin crossover-coupled electron transfer of [M(tacn)(2)](3+/2+) complexes (tacn = 1,4,7-triazacyclononane; M = Cr,...

The role of spin state equilibria on the thermodynamics of electron transfer in [M(tacn)(2)](3+/2+) complexes (tacn = 1,4,7-triazacyclononane; M = Cr, Mn, Fe, Co, Ni) was examined using density functional theory at the B3LYP*/cc-pVTZ(-f) level coupled to a continuum solvation model to afford excellent agreement between computed and experimental redox properties. An intuitive explanation of the previously observed nonperiodic trend in reduction potentials, which display a sawtooth pattern along the first-row transition metal series, is offered utilizing a novel diagrammatic illustration of the relationship between spin state energetics and reduction potentials. This representation leads to a generalized proposal for analyzing and designing nearly isoenergetic spin states of transition metals in a given ligand environment. A new ligand specific parameter alpha that allows for quantifying the differential reduction potential as a function of the metal identity is introduced, and a novel protocol is presented that divides the ligand-metal interactions into primary and secondary characteristics, which we anticipate will be useful for rationally designing the electronics of transition metal complexes in general.