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Suppose $\ce{Zn}$, $\ce{Cd}$ and $\ce{Hg}$ were to form a bond with the same element; say $\ce{ZnCl2}$, $\ce{CdCl2}$ and $\ce{HgCl2}$ Then which of the compounds would be showing most covalent character

I feel like there are two schools of thought with regards to the covalent nature of group 12 elements.

One says according to Fajans' rule covalent character is greatest for smaller cations. Therefore the covalent bond character should be $\ce{Zn}$ > $\ce{Cd}$ > $\ce{Hg}$

The other says (my theory), due to weaker shielding effect of the $\mathrm d$-block there would be more effective nuclear charge for $\ce{Hg}$ than $\ce{Zn}$. So it should be able to cause more distortion and be able have stronger covalent bond. Therefore the order should be $\ce{Zn}$< $\ce{Cd}$< $\ce{Hg}$.

Which one is correct and why?

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The ionic radii (VI coordination) increases from $\ce{Zn^2+}(\pu{0.74 Å)}$ to $\ce{Cd^2+}(\pu{0.95 Å)}$ to $\ce{Hg^2+}(\pu{1.02Å)}$. Applying Fajans' rule, the expected order of covalent character:

$$\ce{ZnCl2} > \ce{CdCl2} > \ce{HgCl2}$$

But consider the following properties:

  • Water solubility order (per $\pu{100 g}$ at $\pu{20 °C})$: $$\ce{ZnCl2}~(\pu{432 g}) > \ce{CdCl2}~(\pu{120 g}) > \ce{HgCl2}~(\pu{7.4g})$$(Source: Wikipedia)

  • Colour intensity comparison due to cations: $$\ce{ZnS}~\text{(white)} < \ce{CdS}~\text{(yellow)} < \ce{HgS}~\text{(black)}$$

Therefore, the correct order of covalent character must be

$$\ce{ZnCl2} < \ce{CdCl2} < \ce{HgCl2}$$

This may be due to increase of effective nuclear charge, for which Slater's rule also fails.

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    $\begingroup$ HgCl2 is a colourless/white solid, unless contaminated by metallic mercury. $\endgroup$
    – Poutnik
    Jul 26 at 10:06
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    $\begingroup$ They are all colorless crystals, not sure where yellow or black (!?) color comes from. It would be nice to see some references for physical properties and solubility data. $\endgroup$
    – andselisk
    Jul 26 at 10:12
  • $\begingroup$ @andselisk Edited, also added references. $\endgroup$
    – Apurvium
    Jul 26 at 10:53
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    $\begingroup$ "(Source: Wikipedia)" You are not serious, are you? $\endgroup$
    – andselisk
    Jul 26 at 10:54
  • $\begingroup$ @andselisk Yes, I am. Though, anyone can edit wikipedia, the information there is near perfect except few cases which can be with any source. Take this: According to "Lange's Handbook of Chemistry- 15E" the solubility data at room temp. are- $\ce{ZnCl2, 395g}$- page 554; $\ce{CdCl2, 120g}$- page 516; $\ce{HgCl2, 7.15g}$- page 533. You can see it is very close to what is in wikipedia. Don't put unnecessary sarcastic comments. You may know much, but should learn how to be polite with strangers! $\endgroup$
    – Apurvium
    Jul 27 at 2:50

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