Osium tetroxide

Osmium tetroxide exists as a pale yellow-brown crystalline solid with a characteristic acrid chlorine-like odor. In fact, theelement name osmium is derived from osme, Greek forodor. OsO₄ is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents, and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). Pureosmium tetraoxide is probably colourless and it has been suggested that its yellow hue is due to osmium dioxide (OsO₂) impurities although osmium (IV) oxide normally exists as a black powder so this may not be true. Osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO₄ penetrate charged cell membranes. OsO₄ is 518 times more soluble in CCl₄ than in water.

With a d⁰ configuration, Os(VIII) is expected to form tetrahedral complexes when bound to four ligands. Tetrahedral structures are seen for the electronically related oxides MnO₄⁻ and CrO₄²⁻.

The osmium of OsO₄ has a formal oxidation state of +8, the highest oxidation state known for a transition metal. The osmium atom has eight valenceelectrons. If one assumes that two electrons are donated by each of the four oxide ligands, the total electron count for the complex is 16, as also seen for the isoelectronic speciespermanganate and chromate.

The high oxidation state of osmium in this compound can be rationalized by comparison of main-group and transition-metal chemistry. Just as the elements in groups 3 through 7 form compounds analogous to those formed by elements in groups 13 through 17 (e.g. TiCl₄ and GeCl₄, VF₅ and AsF₅, CrO₄²⁻ and SeO₄²⁻, etc.), we might expect the elements in group 8 to form compounds analogous to those formed by the noble gases. This is in fact the case, as demonstrated by the existence of compounds like OsO₄ and XeO₄.

OsO₄ is formed slowly when osmium powder reacts with O₂ at 298 K. Reaction of bulk solid requires heating to 670 K.

Os + 2 O₂ → OsO₄

_{References:http://en.wikipedia.org/wiki/Osmium_tetroxide}