Treatment of H2OsBr6 with 1.5 equiv of CsMesH in a mixture of 48% HBr and methanol or ethanol at reflux for 3 h gives a brown precipitate which has the empirical formula “(C5Me5)2Os2Br4H”; actually, this product is the unusual salt [(C5Me5)2OsH]2[Os2Br8] where the cation is the conjugate acid of decamethylosmocene and the anion contains an Os-Os triple bond. This is the first [Os2X82−] salt that has been prepared directly from a mononuclear precursor. Parent peaks due to both the cation and the anion have been observed by electrospray mass spectrometry and negative-ion fast atom bombardment mass spectrometry, respectively. The presence of a hydride ligand in the cation is shown by the singlet in the NMR spectrum at δ −15.65; the NMR spectrum is essentially identical with that of [(C5Me5)2OsH][PF6]. The brown color of [(C5Me5)2OsH]2[Os2Br8] suggests that the anion adopts an eclipsed conformation (D4h symmetry) unlike other known salts of [Os2Br82−], and this conclusion has been confirmed by a single-crystal X-ray diffraction experiment. The two crystallographically independent [Os2Br82‒] anions lie on sites of 3 and 3 symmetry; the Os-Os vectors are correspondingly disordered along each of three mutually perpendicular directions within the ordered cubes of bromine atoms. The Os-Os distances are 2.219(2) and 2.222(2) Å in the two independent anions; the average Os-Br distance is 2.46 Å. The cation adopts a bent metallocene structure with a dihedral angle between the two ring planes of 11°. A byproduct of the reaction of H2OsBr6 with CsMesH in ethanol is the osmium(II) carbonyl complex (C5Me5)2Os2Br2(μ-CO), where the bridging carbon monoxide ligand (vco = 1740 cm−1) is evidently generated from decarbonylation of the solvent. Crystal data for [(C5Me5)2OsH]2[Os2Br8]: trigonal, space group P3, with a = b = 20.732(4) Å, c = 9.780(2) Å, γ = 120°, V = 3640.4(13) Å3, Z = 3, RF [I > 2σ(I)] = 0.0325, and RwF2 [all data] = 0.0925 for 264 parameters refined without constraints against the 2633 observed data or all 3659 unique data, respectively.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry