Chemical activation of pentaosmium carbonyl clusters: Synthesis, structure, and reactivity of [os₅(CO)₁₅(NCMe)] and [os₅H₂(CO)₁₄(NCMe)]; Crystal structures of [os₅(CO)₁₅{P(OMe)₃}] and [os₅H₂(CO)₁₄(PEt₃)]

Simple two-electron oxidation of the salt [N(PPh₃)₂]₂[Os₅(CO)₁₅] with FeCl₃in tetrahydrofuran (thf) in the presence of CO produces the neutral binary pentanuclear cluster [Os₅(CO)₁₆] which upon reaction with 1.1 equivalents of anhydrous Me₃NO in the presence of MeCN at room temperature affords the corresponding activated complex [Os₅(CO)₁₅(NCMe)] in 75% yield. The dihydrido pentanuclear cluster [Os₅H₂(CO)₁₅], which is isoelectronic to [Os₅(CO)₁₆], also reacts with 1.1 equivalents of anhydrous Me₃NO in the presence of MeCN at -78 °C to produce the activated complex [Os₅H₂(CO)₁₄(NCMe)] in 70% yield. Both these activated complexes are believed to be based on a trigonal-bipyramidal arrangement of osmium atoms with an acetonitrile ligand associated with one of the equatorial osmium atoms of the bipyramid. The acetonitrile ligand in both these complexes is labile and can be very easily substituted by other simple two-electron phosphorus-donor ligands L = P(OMe)₃, PPh₃, or P(OPh)₃ to afford [OS₅(CO)₁₅L] and [Os₅H₂(CO)₁₄L] respectively. The structures of [Os₅(CO)₁₅{P(OMe)₃}] and [Os₅H₂(CO)₁₄(PEt₃)] have been determined by a single-crystal X-ray diffraction analysis and contain trigonal-bipyramidal metal frameworks with the phosphorus-donor ligand attached to one of the equatorial osmium atoms of the bipyramid.