# On the stability of methyl mercury

Question: Given that $$\ce{Zn, Hg}$$ are in the same group, how to explain that $$\ce{Hg(CH3)}$$ is stable but $$\ce{Zn(CH3)}$$ is unstable and $$\ce{Hg(CH3)Cl}$$ is stable? $$\ce{Hg(CH3)}$$ seems to have an unpaired electron and hence is unstable.

Background and Thoughts
Some similarities of zinc and mercury: All of $$\ce{M(CH3)Cl, M(CH3)2}$$ and $$\ce{MCl2}$$ are stable, where $$\ce{M=Hg, Zn}$$.
For the causes of the differences of the stabilities of $$\ce{M(CH3)}$$, I can only think of the following reasons:
1. Mercury ion is soft and zinc ion is not very soft. $$\ce{CH3}$$ is very soft. According to the HSAB theory, methyl mercury is stabler than the other one. I'm not very sure of whether HSAB theory can be applied here.
2. Zinc is reactive while mercury is stable.
3. Perhaps it is related to $$d-\sigma$$ conjugation?

For why both $$\ce {Hg(CH3)}$$ and $$\ce{Hg(CH3)+}$$ are stable, I think maybe the reason is strongly related to the fact that $$\ce{Hg-C}$$ bond is covalent.

• It's probably simpler than that and related to oxidation states (Hg +1 is much more prevalent than Zn +1, probably because of relativistic effects). That said, does Hg(CH3) actually exist as such? Is it some kind of dimer Hg2(CH3)2 or even a polymer? – orthocresol May 30 '19 at 11:04