Triosmium dodecacarbonyl


Triosmium dodecacarbonyl is a chemical compound with the formula Os312. 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 Os312 and its lighter analogue Ru312.

Structure and synthesis

The cluster has D3h symmetry, consisting of an equilateral triangle of Os atoms, each of which bears two axial and two equatorial CO ligands. The Os–Os bond distance is 2.88 Â. Ru312 has the same structure, whereas Fe312 is different, with two bridging CO ligands resulting in C2v symmetry.
Os312 is prepared by the direct reaction of OsO4 with carbon monoxide at 175 °C:
The yield is nearly quantitative.

Reactions

The chemical properties of Os312 have been thoroughly examined. Direct reactions of ligands with the cluster often lead to complex product distributions because the inert Os–CO bonds require high temperatures to break, and at such high temperatures the initially formed adducts react further. More successfully, Os312 is converted to more labile derivatives such as Os311 and Os3102 using Me3NO as a decarbonylating agent. Os311 reacts with a variety of even weakly basic ligands to form adducts such as Os311 and Os311. The direct reaction of Os312 with ethylene and pyridine results in degradation of these organic ligands to give the vinyl hydride HOs310 and the pyridyl-hydride HOs310. These products illustrate the stability of Os–H and Os–C bonds.
Os312 is a platform to examine the ways that hydrocarbons can interact with ensembles of metals. For example, the molecule CH3Os310 provided one of the first clear-cut examples of agostic bonding.
From the perspective of bonding, the molecule H2Os310 is noteworthy. In this compound, the two hydride ligands bridge one Os-Os edge. The molecule displays reactivity reminiscent of diborane.