What is the structure of iodide of Millon's base?

Question

In qualitative analysis, $\ce{NH4+}$ ions are confirmed using Nessler's reagent:-

$$\ce{NH4+ + 2[HgI4]^{2−} + 4OH− → HgO·Hg(NH2)I ↓ + 7I^{−} + 3H2O}$$

The brown precipitate formed is called iodide of Millon's base. It is also written as $\ce{3HgO.Hg(NH3)2I2}$ and as $\ce{NH2.Hg2I3}$. (wikipedia).

But its structure is present differently in different sites:-

1. Source 1 - ionic form

2. Source 2 - Here the reaction is given differently:-

$$\ce{([K2]HgI4 <=> 2KI + HgI2) * 2}$$ $$\ce{HgI2 + NH3 -> Hg[NH2]I + NH4I}$$ $$\ce{Hg[NH2]I + HgI2 -> NH2Hg2I3 }$$ $$\ce{[K2]HgI4 + 2NH3 -> NH2Hg2I3 + 4KI + KH4I }$$

structure -

3. Source 3- non ionic form

4. Source 4 - Hydrate form; Here also the reaction is different:- $$\ce{[K2]HgI4 -> 2KI + HgI2}$$ $$\ce{HgI2 + NH3 -> Hg[NH2]I + HI}$$

This site also says that its structure is still uncertain. Various other structure has been proposed:-

So, what is the correct reaction, correct formula and correct structure of iodide of Millon's base?

Answer 1

First, ignore any monomeric formula. The compound is definitely polymeric.

The literature contains hints for existence of two separate lines of compounds. The first one is salts of polyamidomercury: $\ce{[Hg(NH2)]_{n}A_{n}}${1} with mercury-amide chains in the structure, and $\ce{(Hg2N)_{n}A_{n}}$ with structure based on crystoballite {2}{3}

It seems thatn the lines are not distinguished in earlier literature, but recent researchers use the words "Millon's base salts" for $\ce{(Hg2N)_{n}A_{n}}$ only.