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The basicity of pyrazine bound to [MII(CN)5]3- centers (M = Fe, Ru, Os) was studied by spectrophotometric titrations in the pH range 0−7. The overlapping spectral picture is associated with competitive and successive protonation steps. Multiwavelength absorption data were treated by factor-analysis procedures to disclose the pKa for the [M(CN)5pzH]2- species (1.24; 0.41; 0.53 for M = Fe, Ru, Os, respectively). The order of increasing basicity, Fe > Os > Ru, was consistent with the order in the chemical shifts for the β-protons in the pyrazine ring: 8.24 (Fe) < 8.36 (Os) < 8.45 (Ru). pKa values were also obtained for the corresponding [(HNC)M(CN)4pz]2- ions.
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Slep et al. (1999) studied this question.
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