Protonation versus oxidation in the reactions of trifluoroacetic acid with dinuclear osmium(I) complexes: molecular structure of [Os2(MeCo2)2(µ-H)(CO)4(PMe2Ph)2][PF6]

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Abstract
The room temperature reaction between trifluoroacetic acid and the osmium(I) dimer [Os2(MeCO2)2(CO)6] gives the osmium(II) compound [Os(CF3CO2)2(CO)3] as a single osmium product, which is also very easily prepared by the direction reaction of CF3CO2H with [Os3(CO)12] at above 160 °C. However, this acid protonates the more basic derivatives [Os2(MeCO2)2(CO)4L2](L = PMe2Ph, PMePh2, PPh3, or pyridine), to give the hydrido complexes [Os2(MeCO2)2(µ-H)(CO)4L2]+, isolated as the [(CF3CO2)2H]– or [PF6]– salts, which are stable to an excess of CF3CO2H. The X-ray structure of [Os2(MeCO2)2(µ-H)(CO)4(PMe2Ph)2][PF6] indicates that the overall geometry of the molecule is preserved on protonation although the Os Os distance is increased to 3.075(1)Å. Protonation of [Os2(MeCO2)2(CO)4(PMe2Ph)(PPh3)] leads to only a small reduction of the phosphorus–phosphorus nuclear spin–spin coupling constant from 3J= 59.3 to 4J= 46.0 Hz. The direct Os–Os bond is replaced by an interaction via a bridging hydride which substantially preseves the P–P coupling.