[gavindor]

Why are the formulae not matching here for Mercury Hydroxide?

https://pubchem.ncbi.nlm.nih.gov/compound/Mercury-hydroxide

Molecular Formula     H4HgO2
Mercury hydroxide (Hg(OH)2)

The structural formula doesn't show 4 Hydrogens. So the formulae don't match up. Why is that?

And what is  the structural formula version of H4HgO2  ?

Thanks

[Borek]

Looks like something is seriously off.

As far as I am aware mercury hydroxide doesn't exist, precipitate immediately decomposes into HgO. There were some attempts to produce it in very exotic conditions, but it was never isolated.

[gavindor]

Looks like something is seriously off.

As far as I am aware mercury hydroxide doesn't exist, precipitate immediately decomposes into HgO. There were some attempts to produce it in very exotic conditions, but it was never isolated.

Is that the case for Mercury (I) Hydroxide and Mercury (II) Hydroxide? Does one of those exist and not the other? Or do both not exist?

[Borek]

Or do both not exist?

This.

When you attempt to make hydroxide from solution containing Hg₂²⁺ (technically Hg(I)) by adding any strong base, precipitate will immediately undergo disproportionation, producing liquid, metallic Hg⁰ and HgO.

[gavindor]

Though Mercury II Chloride  is  HgCl₂ (So no Hg₂²⁺)

So if you're saying that (also) wouldn't exist, why is that?

[Borek]

Though Mercury II Chloride  is  HgCl₂ (So no Hg₂²⁺)

Yes, but you asked about both Hg(II) and Hg(I) hydroxides, and I already explained earlier what happens with Hg²⁺.

So if you're saying that (also) wouldn't exist, why is that?

"Why" is not a good question here. I can answer "because thermodynamics" but that's really not an answer, as you could ask "why" again. It happens it is this way for mercury, that's all.

[gavindor]

Is it the case that (not just) attempt to make   Mercury I Hydroxide yield HgO. Also, attempts to make  Mercury II Hydroxide yield HgO. i.e. That situation/problem exists for Both Mercury I and Mercury II that attempts to make the Hydroxide, yield  HgO?

[Borek]

Yes, in both cases HgO is the most stable compound that is produced. Note, however, that in the case of Hg(I) it is not the only product - disproportionation produces the same number of moles of Hg⁰ (or Hg(0), no matter how you write it it is the same thing, elemental mercury).

[gavindor]

Thanks. Looking at this one   https://pubchem.ncbi.nlm.nih.gov/compound/119423

They haven't said Mercury (I) Hydroxide there, but is that Mercury (I) Hydroxide?

They've written for the sum formula  H4HgO2

I suppose the condensed structural formula is  (H2O)Hg(H2O)?

[Borek]

I suppose the condensed structural formula is  (H2O)Hg(H2O)?

I would wrote it as Hg(H₂O)₂, but in general yes.

And no, whatever the description is it is wrong, it is not mercury hydroxide. Hydrated mercury atom at best, if something like that even exists (perhaps in some exotic conditions?).

Note that some of the formulas and data are calculated and/or generated by scripts, while they are formally correct now and then they don't reflect reality.

TBH I am surprised NIST database contains such things.

[Corribus]

That's not NIST, it's PubChem. Which is just a database that accumulates reports from the literature (right or wrong).
Also, the heading is incorrect - if you look further down the only name provided by the submitter is mercury hydrate. And there are only two sources recorded.

I guess, it may be from a theoretical calculation.

[Borek]

That's not NIST

Massive brainfart on my side

[Corribus]

Overall though I think you're thinking about it correctly.

Bottom line is that transition metal hydroxides as a class are not particularly stable. Some are more stable than others, of course, but in general the dehydration reaction is highly favored to produce the metal oxide plus water in many cases. "Because of thermodynamics" is as good an answer as any - trends in the properties of transition metals and the compounds they form are often difficult to tease out in any systematic way, likely because the metal d orbitals are numerous, closely spaced, unevenly filled, and quite sensitive to other nearby overlapping orbitals. Spin states and, in some cases further down the table, relativistic effects, also heavily influence physical properties and reactivity. I guess we could hand-wave explanations in isolated cases but such explanations may have limited value. In this case, as I understand, the tendency of mercury (I) ions to dimerize/polymerize is not well-understood, although relativistic stabilization effects have been vaguely implicated.