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

Question

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](http://en.wikipedia.org/wiki/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?

Answer 1

This is an example of hard and soft acids and bases. Something "softness" refers to how diffuse its electron cloud is and how its electrons aren't held very tightly. In the mercury-thiol bond, the mercury is a Lewis acid and the thiol is a Lewis base. Sulfur is a "soft" atom, it's relatively large and not very electronegative. Mercury is also soft due to its very large size. Soft acids prefer to react with soft bases, and vice versa. The oxygen of alcohols, however, is hard. Oxygen is small and very electronegative. So, in an oxygen-mercury bond you have a hard base and a soft acid: not ideal.