The article reported on the diastereomeric dinuclear mixed-valent complexes [(acac)Ru(III)(μ-O)(μ-Pz)Ru(IV)(acac)] (R = H, Me, : ΔΛ, 1a-1c; : ΔΔ/ΛΛ, 2a-2c) and -[(acac)Ru(III)(μ-O)(μ-Iz)Ru(IV)(acac)], (2d) (HPz = pyrazole, HIz = indazole, acac = acetylacetonate). Moreover, the diruthenium(II,II) complexes [(HPz)Ru(II)(μ-O)(μ-Pz) Ru(II)(HPz)] (3a) and [(HIz)Ru(II)(μ-O)(μ-Iz)Ru(II)(HIz)] (3d) were presented. The analogous form of 3a, , [(HPz)(Pz)Ru(III)(μ-O)(μ-Pz)Ru(III)(Pz)(HPz)], was previously reported. Single crystal X-ray structures of 1a-1c/2a-2d and representative 3a showed their molecular forms, including the diastereomeric nature of the former. The Ru-O-Ru angle decreased appreciably on switching from doubly bridged 1 and 2 (128-135°) to triply bridged 3a (114°). Both series of complexes displayed rhombic symmetry in their EPR spectra, with and being very similar for 1a-1c with an almost axial look. The mixed-valence complex with a Ru(III)Ru(IV) ( = 1/2) state of 1 and 2 would lead to iso-valence complexes of Ru(III)Ru(III) and Ru(IV)Ru(IV) with an EPR inactive state by one electron redox reaction. On the other hand, metal based {Ru(II)Ru(II)/Ru(II)Ru(III), 3a/3a} and terminal ligand (HPz/HPz, 3a/3a) based redox processes displayed anisotropic and free radical EPR, respectively. An IVCT (intervalence charge transfer) band was found for the delocalised mixed valent 1 and 2 {Ru(III)Ru(IV)} or 3a {Ru(II)Ru(III)} in the NIR region. The intense metal-to-ligand charge transfer (MLCT) transitions of 1-3 in the visible region varied systematically as a function of the metal oxidation state.
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