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Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)x(bpy)3–x]2+(CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x= 1–3, abbreviated as 12+, 22+, and 32+) undergo four (12+) or five (22+and 32+) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+and 32+being separated by 150–210 mV; reduction of the unsubstituted bpy ligand in 12+and 22+occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. −45 cm–1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•–, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1–, and 2–exhibit a prominent band attributable to the CN-Me-bpy•–moiety between 6000 and 7500 cm–1, whereas bpy•–-based absorption occurs between 4500 and 6000 cm–1; complexes 2+, 3+, and 30also exhibit a band at ca. 3300 cm–1due to a CN-Me-bpy•–→ CN-Me-bpy interligand charge-transfer transition. In the UV–vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by −38 cm–1and 5–7-fold-enhanced relative to the ground state.

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+(where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means.