Molecular Structure, Spectroscopy, and Photoinduced Kinetics in Trinuclear Cyanide Bridged Complex in Solution: A First-Principles Perspective

We investigate the molecular structure of the solvated complex, [(NC)₆Fe–Pt­(NH₃)₄–Fe­(CN)₆]^4–, and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt­(IV) metal center resulting from Fe ← Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt­(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ← Pt)