Characteristic vibrational frequencies of osmium(II) nitrosyl complexes probed by Raman spectroscopy and DFT calculations

Solid-state Raman spectra at variable temperature were measured for seven six-coordinate osmium(II) nitrosyl complexes. Five of them share a common (n-Bu4N)[Os(NO)Cl-4(L)] stoichiometry with L being an azole-type ligand either cis or trans to the NO ligand. The azole ligands are Hpz = 1H-pyrazole (compounds 1 and 2 are cis and trans isomers), Hind = 1H-indazole (compounds 3 and 4 are cis and trans isomers) and Him = 1H-imidazole (compound 5, cis isomer only). The two remaining compounds also contain nitrosyl and chloride ligands, but the L ligand is a bidentate oxalate (ox, compound 6) and glycinate (gly, compound 7). All complexes were previously crystallographically characterized and feature a NO ligand coordinated to the metal center through the nitrogen atom. DFT calculations at the B3LYP/MWB60(Os)/6-31G*(other atoms) level of theory are presented for the cis and trans isomers of (n-Bu4N)[Os(NO)Cl-4(Hpz)] (1 and 2), allowing to reliably assign the three characteristic vibrations nu(NO), nu(Os-NO) and delta(OsNO). The temperature dependence of these Raman bands and vibrational frequencies is presented and discussed.