# Why do thiols have such a propensity for bonding with mercury?

Historically, thiols $\ce{-SH}$ were known as "mercapto-" due to their strong propensity to bond with mercury. Certain drugs still carry this designation, though mercaptopurine has an $\ce{=S}$ group instead of a thiol.

What makes the thiol group bond so strongly with mercury in particular? Why don't alcohols make such a strong bond with $\ce{Hg}$?

If it's known, was there any historical reason why it was important to be able to "capture" mercury?

• "...though mercaptopurine has an $\ce{=S}$ group instead of a thiol." - that would be the tautomerism at work. See this article for instance. – user95 Jul 7 '12 at 16:59
• @J.M. Ah, I see, definitely didn't think of that. Thank you, I'll have to download that later. – jonsca Jul 7 '12 at 17:04

Well, sulfur in general, and thiolates in particular, are known for bonding very strongly to gold, lead and mercury. In the particular case of mercury, which was known very early in the history of chemistry (indeed since the times of alchemy), this is the reason thiols are often called "mercaptans", from the Latin mercurium captans (capturing mercury).

In Pearson's HSAB theory (hard and soft acids and bases), the reason the S-Hg bond is be stronger than the O-Hg can be explained because S2- is a soft basis and Hg(II) is a soft acid, making a good fit… while O2- is harder.

Finally, regarding the historical use of thiols to bind mercury (and lead to), let's just say a chelating agent for mercury and other toxic compounds was quite useful as an antidote. You can read a very nice book on the historical use of mercury and lead in John Emsley's The Elements of Murder.

• As an additional note, Hazard Information sheets for mercury spills still recommend using either sulphur or a mixture of sand and sulphur to clean up anything you can't pipette. For example. – Aesin May 7 '12 at 3:11
• A fair amount of the chelating agents currently used in modern medicine possess a mercapto group. Off the top of my head, there's dimercaprol (BAL, British anti-lewisite), penicillamine, succimer (DMSA, dimercaptosuccinic acid), tiopronin... – user95 Jul 7 '12 at 17:03