Triosmium dodecacarbonyl is a chemical compound with the formula Os<sub>3</sub>(CO)<sub>12</sub>. This yellow-colored metal carbonyl cluster is an important precursor to organo-osmium compounds. Many of the advances in cluster chemistry have arisen from studies on derivatives of Os<sub>3</sub>(CO)<sub>12</sub> and its lighter analogue Ru<sub>3</sub>(CO)<sub>12</sub>.
The cluster has D<sub>3h</sub> symmetry, consisting of an equilateral triangle of Os atoms, each of which bears two axial and two equatorial CO ligands. Each of the three osmium centers has an octahederal structure with four CO ligands and the other two osmium atoms.
The OsâÂÂOs bond distance is 2.88 à(288 pm). Ru<sub>3</sub>(CO)<sub>12</sub> has the same structure, whereas Fe<sub>3</sub>(CO)<sub>12</sub> is different, with two bridging CO ligands resulting in C<sub>2v</sub> symmetry. In solution, is fluxional as indicated by <sup>13</sup>C NMR measurements. The barrier is estimated at 70 kJ/mol
Os<sub>3</sub>(CO)<sub>12</sub> is prepared by the direct reaction of OsO<sub>4</sub> with carbon monoxide at 175 ðC under high pressures:
The yield is nearly quantitative.
Many chemical reactions of Os<sub>3</sub>(CO)<sub>12</sub> have been examined. Direct reactions of ligands with the cluster often lead to complex product distributions. Os<sub>3</sub>(CO)<sub>12</sub> converts to more labile derivatives such as Os<sub>3</sub>(CO)<sub>11</sub>(MeCN) and Os<sub>3</sub>(CO)<sub>10</sub>(MeCN)<sub>2</sub> using Me<sub>3</sub>NO as a decarbonylating agent:
Os<sub>3</sub>(CO)<sub>11</sub>(MeCN) reacts with a variety of even weakly basic ligands to form adducts.
Purging a solution of triosmium dodecacarbonyl in boiling octane (or similar inert solvent of similar boiling point) with H<sub>2</sub> gives the dihydride Os<sub>3</sub>H<sub>2</sub>(CO)<sub>10</sub>:
Osmium pentacarbonyl is obtained by treating solid triosmium dodecacarbonyl with 200 atmospheres of carbon monoxide at 280-290 ðC.