[mrorganiclover]

Chelation therapy is often used for treatment of metal poisoning. The drug administered acts as a chelating ligand toward the toxic metal ion, forming a stable complex that can be excreted intact from the body. Which of the following compounds would be effective for treatment of mercury poisoning? Explain your
answer.

Good day all

Please see the two molecules on the attached image.

From the research I have done, I have chosen the molecule that contains the two SH groups. This is because S is hypervalent, thus allowing a bond to be made to the Hg whereas the OH groups on the other molecule all have a full octet meaning no new bonds can be formed. Please don't hesitate to correct me if I'm wrong.

[kriggy]

Your answer is right, however your reasoning is wrong.
Both compounds can form coordination bonds with metals when the donating atom (in this case O or S) donates its nonbonding electron pair into free orbitals of metal forming a chemical bond.
http://en.wikipedia.org/wiki/Coordination_complex

Anyway, the reason why the dimercaprol (also known as " British anti-Lewisite", cmpd. a) will coordinate Hg while the other compound will not is due to HSAB theory.
http://en.wikipedia.org/wiki/HSAB_theory
This theory is widely used in coordination chemistry where it explains the reactivity of metals (acids) and ligands (bases). It divides those acids and bases into two groups - hard and soft (and intermediate). THe point is that hard acids prefer to react with hard bases and soft acids prefer to react with soft bases.
Hg is soft acid but suphur is intermediate base while oxygen is hard base so the cmpd. b will not reacti while cmpd. a will which will then make the Hg basicaly harmless and able to be eliminated from body (note that cmpd. a was used as chelating agent for acute metal poisoning).
http://en.wikipedia.org/wiki/Chelation_therapy

[mrorganiclover]

Thank you very much

[mrorganiclover]

Thanks for taking the time to post those links also.