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The Electronic Structure of Oxidized Complexes Derived from cis-[RuII(bpy)2(H2O)2]2+ and the Mechanism of Its Photoisomerization

Planas, N.; Vigara, L.; Cady, C.; Miró, P.; Huang, P.; Hammarström, L.; Styring, S.; Leidel, N.; Dau, H.; Haumann, M.; Gagliardi, L.; Cramer, C. J.; Llobet, A.
Inorg. Chem. 2011, 50, 11134 (doi:10.1021/ic201686c).

The geometry and electronic structure of cis-[RuII(bpy)2(H2O)2]2+ and its higher oxidation state species up formally to RuVI have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low spin configurations are favored for all oxidation states. Thus cis-[RuIV(bpy)2(OH)2]2+ (d4) has a singlet ground state and is EPR silent at low temperature while cis-[RuV(bpy)2(O)(OH)]2+ (d3) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[RuII(bpy)2(H2O)2]2+ to its trans-[RuII(bpy)2(H2O)2]2+ isomer has been fully characterized through quantum chemical calculations. The process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.