Osmium is a rare metal, but its oxide is so useful it finds its way into chemistry in all sorts of places.
Its widest use is probably the addition of two vicinal hydroxyl groups where a double bond used to be. Barry Sharpless pioneered its use in the presence of a chiral amine to make hydroxylation selective and faster. And not too long ago, we realized that any amine would speed it up - if you want a racemate, just use an achiral amine.
OsO4 is also very useful as a stain, because it'll leave behind a heavy metal (and very dark area) - useful for a contrast agent in both optical and electron microscopy.
The unfortunate thing about OsO4 is that it's a bit more dangerous than your average reagent. It's a solid and looks pretty innocuous (disappointing, even, especially considering its exorbitant price). It has a measurable vapor pressure, however, and it can actually work its prodigous stain magic upon your corneas, which, as you can imagine, is an undesirable state of affairs.
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Really useful to increase the number of terminal hydroxyl groups on PEO dendrimers. You just need to functionalize with some allyl moieties, check their number via NMR, and here comes OsO4!!! NMR will then give you the yield.
A few years ago a terrorist cell hatched a plot to gas the London subways with the stuff
The BBC article
I'm sure this is the stuff our lecturers said would coat your eyeballs with Osmium. They liked to tell us nice helpful things like that, possibly to help us remember things.
Wish it would penetrate biological tissue more readily. I have had several great micrograph shots ruined by a distinct line where OsO4 decided to end it's journey.
Toxic as hell and one of the strongest oxidisers. Still useful but with so many reagents out there there could be an easier to use alternative in most cases.