Hg2+ Structure, Characteristics:17 Quick Complete Facts

In this article, we should discuss the Hg2+ structure and its important facts in detail. Let us start the article with the Hg2+ electronic configuration.

Hg2+ structure consist of ten 5d electrons. It is a post-transition element or borderline element. The electronic configuration of Hg is [Xe]4f145d106s2.  But the electronic configuration of the Hg2+ structure is [Xe]4f145d10 two electrons from 6s orbitals are removed and gain the noble liquid configuration.

Although the Hg2+ structure is a cation due to the complete “d” orbital it gains extra stability. The Hg2+ structure Is very toxic to a human being. It causes different health issues.

Some facts about the Hg2+ structure

Hg2+ structure comes from the reduction of Hg metal. When Hg released two electrons then Hg2+ Is produced. The reduction potential of thIs process Is a very low negative value. In the liquid, Hg released 2 electrons in an aqueous medium.

Hg – 2e = Hg2+ , E0 = -0.85V

There is an equilibrium that exists between Hg22+ and Hg2+. Because in an aqueous solution, the Hg22+ ion readily tends to disproportionate into Hg(II) and Hg. Because it can disproportionate its two oxidation states as Hg(I) is in an intermediate oxidation state.

The equilibrium between Hg(I) and Hg(II) is very delicate, as one may readily appreciate from the following E0 values:

Hg(I) and Hg(II) Equilibrium

Subtracting the second from the first we get,

Hg22+ = Hg(l) + Hg2+, E0 = -0.115V

This shows that in any solution containing Hg(I), there will be rather more than 1% Hg(II) in equilibrium.

CFT of Hg(II)

The complexes of Hg(II) will not gain any stabilization from the ligand field (LFSE) as its d orbital is filled with electrons.

At the same time, the stability of the d10 core makes the Hg(II) ions reluctant to back-bonding and we don’t find its complexes with π-acceptor ligands like Co, NO, or alkenes. The cyanide of Hg(II) is stabilized mainly by ϭ-bonding alone.

Similarly, the filled d-shell does not allow electron acceptance into these orbitals, so even good Π-donor ligands like cyclopentadienide also form ϭ-bonded complexes with the metals.

Since the complexes of this metal cannot gain any stabilization under their geometry (LFSE =0), the stereochemistry is determined by the steric requirements of the ligands and the size and polarizing power of the cation.

Two ligands approach the Hg2+ ion from two sides along the Z-axis, and the d-electron population will be deformed or pushed in the XY plane. The enhanced electron density will now repel other ligands approaching this plane.

In keeping with its similarities to the main group II elements, Hg(II) is a class-b metal as it forms stable complexes with mainly P and S donor ligands.

Read more about Hexanol Structure and Characteristics

Hg(II) complex of weak field ligands

HgO which adopts only the zinc blende form may be yellow or red depending on the particle size. The red form is obtained by slow heating of mercury in O2 at about 3500C or by heating Hg(NO3)2. The yellow form is precipitated by alkali from an aqueous solution of Hg(II).

Hg(NO3)2 + 2KOH = HgO↓ +2KNO3 + H2O

Both forms have the same zig-zag chain structure with virtually linear O-Hg-O units.

Mercury (II) hydroxide is unknown. When aqueous solutions containing Hg(II) structure are reacted with alkali, yellow HgO is precipitated.

The less stability of HgS, it is directly converted to Hg on heating. The fluoride of Hg(II) is purely ionic having a high melting point.

Mercuric Chloride, HgCl2 is a corrosive sublimate prepared by heating Hg in chlorine or by heating a dry mixture of mercuric sulfate and NaCl when the mercuric chloride is obtained as a white sublimate.

It is sparingly soluble in cold water but freely in hot water, mainly undissociated. It is more soluble in methanol and ether.

On boiling with an aqueous ammonia solution, mercuric chloride gives an “infusible white precipitate” of Hg(NH2)Cl which is hydrolyzed by digestion to yield “Chloride of Millon’s base”, NH2HgO.HgCl.

HgCl2 + 2NH3 = Hg(NH2)Cl + NH4Cl

2Hg(NH2)Cl + H2O = (NH2)HgOHgCl + NH4Cl

HgCl2 reacts with gaseous ammonia to form the “fusible white precipitate” of HgCl2.2NH3.

Mercuric chloride oxidizes stannous chloride to stannic chloride, a reaction commonly used in the traditional volumetric estimation of iron(III) after reduction with SnCl2.

2HgCl2 + SnCl2 = Hg2Cl2 + SnCl4

Hg2Cl2 + SnCl2 = 2Hg + SnCl4

Crystalline complex salts K[HgCl3] and Na2[HgCl4] may be obtained by reaction solution of the alkali metal chlorides with HgCl2.

Potassium iodide gives mercuric chloride an initial yellow precipitate of HgI2 which rapidly turns red and finally dissolves in excess KI and K2HgI4.

Nessler’s reagent is an alkaline solution of K2HgI4 which gives a brown precipitate with ammonia which is a detection test of NH3.

2K2HgI4 + NH3 + 3KOH = Hg2NI.H2O + 7KI + 2H2O

Actually, Hg2NI.H2O Is called the iodide of Millon’s base.

Mercuric nitrate, Hg(NO3)2.H2O deposits as colorless deliquescent crystals from a solution of mercury in hot concentrated HNO3. It is soluble in water containing nitric acid but otherwise gets extensively hydrolyzed and in dilute solution breaks up completely into HgO and HNO3.

HgSO4 crystallizes in silvery plates from a solution of Hg concentrated sulfuric acid. It is hydrolyzed by water to a lemon-colored basic sulfate.

Read more about H2CO lewis structure

Hg(II) complex of strong field ligands

Hg(CN)2 is formed by the reaction between alkali cyanide and mercury (II) solution – the resulting solution on concentration yields colorless crystals. It Is fairly soluble in water but not in ethanol. Hg(CN)2 is practically undissociated in solution as fails to give any precipitate with KOH or KI solutions.

It decomposes on heating to Hg and (CN)2. With excess cyanide ion, complexes of the type [Hg(CN)3] and [Hg(CN)4]2 are formed.

Hg(SCN)2 is formed as a sparingly soluble white precipitate from the reaction of Hg(II) and SCN ions in solution. The compound is practically undissociated in solution as indicated by its vanishingly small electrical conductance.

Excess thiocyanate forms soluble complexes [Hg(SCN)3] and [Hg(SCN)4]2-.

When ignited in the air, pellets of Hg(SCN)2 swell enormously into a curly snake-like residue of spongy ash and hence its use as fireworks (Pharaoh’s serpent). The final product Is some polymerized cyanogen compound.

Crystalline Hg(SCN)2 is built up of distorted octahedral units with bridging SCN groups.

Mercuric Fulminate, Hg(ONC)2 Is obtained as a white precipitate by warming a solution of mercuric nitrate with an excess of nitric acid and methanol. The compound explodes hence is used in making detonators.

The formation of Hg(II)-N covalent bonds in the reaction of Hg(II) with aqueous ammonia.

Hg2+ + 2NH3 = [Hg-NH2]+ + NH4+

Actually, such reactions give a variety of products depending upon the conditions. The reaction between HgCl2 and aqueous NH3 produces a number of products in which the hydrogens of NH3 are substituted by Hg. Three main reactions may be identified:

HgCl2 +  2NH3 = Hg(NH3)2Cl2 (s)                 “fusible white precipitate”

Hg(NH3)2Cl2 = Hg(NH2)Cl + NH4Cl            “infusible white precipitate”

2Hg(NH2)Cl + H2O = [Hg2NCl(H2O)] + NH4Cl  “chloride of Millon’s base”

In the presence of excess NH4Cl, HgCl2 reacts with boiling ammonia solution to form the white precipitate of Hg(NH3)Cl2 – the subsequent reactions are suppressed by the presence of NH4+.

The same compound is also formed by a reaction between HgCl2 and NH3(g). the precipitate melts when heated undergoing decomposition and hence was named the fusible white precipitate.

X-ray studies reveal that the compound consists of linear NH3-Hg-NH3 units inserted in a cubic lattice of Cl- ions, each Hg(II) attaining six-coordination from four Cl- and two NH3 in a distorted octahedral arrangement.

1.    Hg2+ lewis structure formal charge

The formal charge Is applied in the molecule but the cation species, we can also predict the formal charge for the Hg2+ structure.

we use the formula to calculate the formal charge for the Hg2+ structure is,

F.C. = Nv – Nl.p. -1/2 Nb.p. Where Nv is the number of electrons in the valence shell or outermost orbital, Nl.p is the number of electrons in the lone pair and Nb.p  is the total number of electrons that are involved in the bond formation only.

In the Hg2+ structure, there are only two valence electrons present and no lone pair present, and as it exists in elemental form so no bond pair electrons are present.

So, the formal charge of the Hg2+ structure Is, 2-0-0 = 2

From the value of the formal charge of Hg2+, it Is evident that it is charged particle and the value Is +2 as it contains dication.

2.    Hg2+ valence electrons

To predict the Hg2+ structure valence electrons we should count the valence electrons for hg and then predict the valence electrons for the Hg2+ structure.

Hg2+ Valency

The electronic configuration of the Hg2+ structure is, [Xe]4f145d10, so it has a vacant 6s orbital and can bind two ligands to form a stable complex. The cation itself is stable because it adopts a noble liquid configuration due to a filled 5d orbital.

 But due to two positive charges, it can bind two anions and the valency will be two for the Hg2+ structure.

3.    Hg2+ lewis structure octet rule

Although the Hg2+ structure is from the d block element it follows the octet rule. It has a fully d orbital with ten electrons.

Hg2+ structure
Hg2+ Octet

The electronic configuration of Hg2+ structure is [Xe]4f145d10. So, it has already ten electrons in the d orbital. We know the d orbital contains a maximum of ten electrons as it contains five subshells and each subshell can accumulate a maximum of two electrons.

The d block element contains 18 electrons to complete its octet. There are two electrons in the 5s orbital, six electrons in the 5p orbital, and ten electrons in the 5d orbital. So, it has 18 electrons in its valence shell and completes its octet as it is a d block transition element.

4.    Hg2+ lewis structure lone pairs

In the Hg2+ structure, it is an elemental form so all the electrons present in the valence shell are present as a paired form so no need for lone pairs or bond pairs.

Hg2+ structure completes its vacant d orbital by ten electrons. There are two electrons in the 6s orbital in the Hg, but those two electrons are removed for the Hg2+ structure. Actually, in the Hg2+ structure, there are two positive charges are present and there are no lone pairs present over the Hg2+ structure.

In the Hg2+ structure, there is no bonding present so, we cannot predict how many electrons are present in the valence shell after bond formation, so it’s difficult to predict the lone pairs of dication.

5.    Hg2+ solubility

Hg2+ structure soluble in,

  • Chloride
  • Nitrate
  • Chromate

6.    Is Hg2+ soluble in water?

No, the Hg2+ structure Is insoluble in water.

It is very heavy-ion and it is a group IA cation.

7.    Is Hg2+ paramagnetic or diamagnetic?

Hg2+ is diamagnetic in nature.

All the d electrons in the Hg2+ structure are in paired form, and no unpaired electrons are present, so it is diamagnetic.

8.    Is Hg2+ a lewis acid?

Hg2+ can behave as lewis acid.

The 6s orbital for the Hg2+ structure is now vacant and it can take electrons so it behaves as lewis acid.

9.    Is Hg2+ a denaturing agent?

Yes, Hg2+ is a denaturing agent.

It can denature the primary protein structure so it is a denaturing agent.

10.  Is Hg2(no3)2 soluble in water?

Yes, Hg2(NO3)2 is soluble in water.

The ionized form of Hg2(NO3)2 is a nitrate which can be soluble in water.

11.  Is Hg2+ monatomic or polyatomic?

Hg2+ is diatomic.

Because there is only tow cation present in the structure.

12.  Is Hg2(clo3)2 soluble?

Yes, Hg2(ClO3)2 is soluble in water.

There is a hydrophilic part is present which is ClO3, which can be easily soluble in water.

13.  Is Hg2(c2h3o2)2 soluble in water?

No, Hg(C2H3O2)2 is insoluble in water.

Due to the presence of the hydrophobic part, as C2H3O2  is an organic moiety so, it is insoluble in water.

14.  What is Hg2(cr2o7)?

Fluorescent metal-organic framework.

Due to the presence of fluorescent parts, it can behave as a light-harvesting agent.

15.  Is Hg2+ ionic or covalent?

It is covalent in nature.

The presence of a ten d electron makes Hg2+ covalent in nature.

16.  Is Hg2+ hard or soft?

Hg2+ Is soft acid but mostly borderline acid.

Due to the presence of ten d electrons, the size of Hg2+ is lower but the charge potential is also low so making it soft acid. it is preferred to bind a soft base.

17.  Is Hg2(no3)2 an electrolyte or nonelectrolyte?

Yes, Hg2(NO3)2 is an electrolyte.

Because in an aqueous solution it’s ionized and formed nitrate which can carry electricity making the molecule an electrolyte.


Hg2+ structure is one of the covalent cations and due to higher d electron, it is soft acid and most inner to organometallic ligands. But it can form a bond with suitable ligands and it is not health-friendly in nature.

Biswarup Chandra Dey

Chemistry is not all about reading line by line and memorize, it is a concept to understand in easy way and here I am share with you the concept about chemistry which I learn because knowledge is worth to share it.

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