Synthesis and Electronic Structure of Pentacyanoosmate(II) Complexes with N-Heterocyclic Ligands
The series of complexes [OsII(CN)5L]n-, with L = pyridine or pyrazine derivatives, were prepared in aqueous solution and, in some cases, as sodium or potassium salts. The main feature in the UV-visible spectra is the appearance of an intense, asymmetric MLCT band, split under spin-orbit coupling. Th...
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| Autores principales: | , , |
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| Formato: | JOUR |
| Acceso en línea: | http://hdl.handle.net/20.500.12110/paper_00201669_v35_n21_p6327_Slep |
| Aporte de: |
| Sumario: | The series of complexes [OsII(CN)5L]n-, with L = pyridine or pyrazine derivatives, were prepared in aqueous solution and, in some cases, as sodium or potassium salts. The main feature in the UV-visible spectra is the appearance of an intense, asymmetric MLCT band, split under spin-orbit coupling. The energies and intensities of the MLCT bands decrease and increase, respectively, with the electron-acceptor ability of L, and strong solvatochromic energy shifts are observed in different organic media. The Os(II) complexes can be oxidized chemically or electrochemically to the Os(III) species; the latter show typical LMCT bands in the visible region, independent of L. The redox potentials for the OsIII.II couples (range 0.6-1.0 V (NHE)), shift positively when L becomes more electron-withdrawing or less basic. Reduction potentials for the bound and free Mepz+ ligand showed similar values, ca. -0.53 V (NHE), for the three [M(CN)5L]n- complexes, suggesting similar back-bonding abilities of Fe, Ru, and Os toward a given L ligand; this is confirmed by the linear plots with unit slope obtained for the energy of the MLCT bands of the [M(CN)5L]n- complexes (M = Fe, Ru) against the values for the [Os(CN)5L]n- complexes. The IR spectra show intense and weak bands at ca. 2050 and 2100 cm-1, associated with equatorial and axial cyanide stretchings, respectively. The dissociation rate constant for pyrazine release from the [Os(CN)5pz]3- ion shows a saturation kinetic behavior, typical of dissociative mechanisms found for the iron and ruthenium analog complexes; the specific dissociation rate constant, k-pz = 2.0 × 10-8 s-1 (25 °C, I = 0.5 M), is about 3 and 4 orders of magnitude slower than the values found for [Ru(CN)5pz]3- and [Fe(CN)5pz]3-, respectively; this is ascribed mainly to the strong σ interaction in the Os-L bond. |
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