Photochemical behaviour of ion pairs of the type {[Cr(en) 3] 3+; [M(CN) 6] 4−} (MFe, Ru, Os) as a first example of second-sphere photoeffects of chromium(III) amine complexes

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Ion pairs of the constitution {[Cr(en) 3] 3+; [M(CN) 6] 4−} show, in an aqueous buffer solution, second-sphere charge transfer (SSCT) absorption bands at 342 nm (MFe), 290 nm (MRu) and 303 nm (MOs) additionally to the spectra of the individual components. Two possible mechanisms of the optical electron transfer process involving the initial formation of low spin (t 2g 4) and high spin (t 2g 3 e g 1) [Cr(en) 3] 2+ respectively the discussed in order to explain the rather high energies of the SSCT absorption maxima. The ion pairs under discussion undergo photosubstitution reactions upon ligand field excitation (λ irr=450 nm) of the chromium complex. Binuclear complexes [(H 2O(en) 2CrNCM(CN) 5] − (MFe, Ru, Os) are formed, which are distinguished by intense metal-to-metal charge transfer absorption bands in the UV spectral region. These binuclear complexes do not undergo further photoreactions. Photoredox reactions, indicated by the formation of [M(CN) 6] 3− (MRu, Os), are initiated upon excitation of the SSCT transition (λ irr=320 nm) within the ion pair compounds. Additionally, also photosubstitution is observed upon short-wavelength irradiation.