Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule

The complexes <i>trans</i>-[Ru^III(NH₃)₄(4-pic)­(H₂O)]­(CF₃SO₃)₃ (<b>1</b>) and [Ru^III(NH₃)₅(4-pic)]­(CF₃SO₃)₃ (<b>2</b>) were isolated and studied experimentally by electron paramagnetic resonance (EPR) and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography and theoretically by ligand-field theory (LFT) and density functional theory (DFT) calculations. Complex <b>1</b> is reported in two different crystal forms, <b>1a</b> (100 K) and <b>1b</b> (room temperature). EPR and UV–vis spectroscopies suggest that aqua ligand interaction in this low-spin ruthenium­(III) complex changes as a function of hydrogen bonding with solvent molecules. This explicit water solvent effect was explained theoretically by DFT calculations, which demonstrated the effect of rotation of the aqua ligand about the N_pic–Ru–O_aq axis. The UV–vis spectrum of <b>1</b> shows in an aqueous acid solution a broad- and low-intensity absorption band around 28 500 cm^–1 (ε ≈ 500 M^–1 cm^–1) that is assigned mainly to a charge-transfer (CT) transition from the equatorial ligands to the Ru β-4d_<i>xy</i> orbital (β-LUMO) using DFT calculations. The electronic reflectance spectrum of <b>1</b> shows a broad and intense absorption band around 25 500 cm^–1 that is assigned to a CT transition from 4-picoline to the Ru β-4d_<i>xz</i> orbital (β-LUMO) using DFT calculations. The t_2g⁵ set of orbitals had its energy splitting investigated by LFT. LFT analysis shows that a rhombic component arises from <i>C</i>_2<i>v</i> symmetry by a simple π-bonding ligand (H₂O in our case) twisting about the trans (<i>C</i>₂) axis. This twist was manifested in the EPR spectra, which were recorded for <b>1</b> as a function of the solvent in comparison with [Ru­(NH₃)₅(4-pic)]³⁺ and [Ru­(NH₃)₅(H₂O)]³⁺. Only <b>1</b> shows an evident change in the <b>g</b>-tensor values, wherein an increased rhombic component is correlated with a higher nucleophilicity (donor) solvent feature, as parametrized by the Abraham system