The Wizard of Os

This post has been inspired by a reaction that I am too terrified to try. The reaction is called an oxidative cleavage and is done using osmium tetroxide, or OsO4. I was once referred to as "osmiphobic" by one of my committee members during my candidacy exam. So lets take a look at this element called Osmium and then discuss why I am so afraid to work with OsO4. 

Osmium is element 76 on the periodic table. It is found in the transition metal section (the middle part) at the bottom of the same column as Ruthenium (Ru), element 44, and Iron (Fe), element 26. Osmium has an atomic mass of 190.23 g/mol. What is so neat about this metal? Well, unlike most metals, Os isn't grey (or silver). It is actually VERY faint blue colour. Other metals that are not grey or silver are gold (Au), element 79, and copper (Cu) element 29. 

Osmium is actually the hardest pure metal on the Brinell scale. http://en.wikipedia.org/wiki/Hardnesses_of_the_elements_%28data_page%29 Osmium is not the hardest element (carbon: in its diamond allotrope) or even the hardest material, these are mixtures of various elements. But in its pure form, it is the hardest metal. And metals make up the majority of the periodic table. Its hardness has made osmium useful for writing implements and is therefore found in the nibs of fountain pens. It was also used in the tips of needles for phonographs, which are (were) subject to significant ware. 

Interestingly, compared with other metals in this area of the periodic table, gold, rhenium, platinum, that are oxidation resistant, osmium will oxidise slowly in air, forming OsO4. Which brings us to this compound I am freaked out about working with. 

OsO4 is a solid material, but it is actually quite volatile. It will sublime (a process whereby a solid is converted directly to a gas without proceeding through a liquid state)  at room temperature. This is HIGHLY TOXIC! As a matter of fact, a small, sealed ampule of OsO4 obtained from Sigman-Aldrich has "highly toxic" or "very toxic" written on the label in no less than four places, with the accompanying "skull and cross bones" symbol of death. Have a look at the Materials Safety Data Sheet (MSDS) for this compound: http://www.2spi.com/catalog/msds/msds02595.html it is NOT pleasant. It has an extremely low LD50 (the amount necessary to cause the death of half of a sample set population-usually conducted on mice, rats, or guinea pigs). It is rumoured to have a smell something akin to that of ozone; however, if inhaled at levels well below that required to register a smell, this compound can cause pulmonary edema, resulting in death, so I am not anxious to confirm the smell of this compound.

Now I have worked with some pretty nasty chemicals: lead tetracetate, sodium cyanide, sodium azide, dimethyl aminopyridine (DMAP) the list goes on and on. Why I feel this compound is so different is because of this volatility. In its vapour phase, it is much easier to penetrate the safety barriers set up. For example: one should not wear typical safety glasses, but seal goggles because of the ease in which a vapour gets past safety glasses and the fact that one of the main target organs for this compound is the eye. It is much harder to protect against a vapour. 

There is a related compound for oxidative cleavage called ruthenium tetroxide, RuO4. Ruthenium is right above osmium on the periodic table. The difference between RuO4 and OsO4 in this reaction is that that RuO4 oxidises to the carboxylic acid-the highest oxidation state of carbon, while OsO4 oxidises only to the aldehyde-the second highest oxidation state of carbon. This is because of a neat little trend of transition metals. Anywhere else on the periodic table, the bonds get weaker as one moves down the table. But in the transition metals have the opposite trend, the bonds get stronger. This is why most catalysts are still made from the more toxic transition metals further down, rather than the less toxic transition metals in the first row, like nickel, copper, and iron. The bonds are too weak and fall apart easy. The metals in the last row are often not used because the bonds are too strong. Leaving the middle row as the "Goldie Locks" row-the bonds are just right. In the case of OsO4 though, those strong bonds work to our advantage by preventing over-oxidation. 

So that is the story on osmium and osmium tetroxide. I am sure one day I will get up the courage to work with this chemical, making sure to practice safe chemical handling techniques when doing so. I have, after all, worked with other equally nasty chemicals. 

Reference:

Gray, T. The Elements 2009 Black Dog & Leventhal Publishers Inc., New York, NY.