Question

Which of the following species is not stable?
A. ${{\left[ Si{{F}_{6}} \right]}^{-2}}$
B. ${{\left[ GeC{{l}_{6}} \right]}^{-2}}$
C. ${{\left[ Sn{{\left( OH \right)}_{6}} \right]}^{-2}}$
D. ${{\left[ SiC{{l}_{6}} \right]}^{-2}}$

Answer 1

The central metal atoms in the options belong to p-block elements and they can extend their covalencies beyond that of the octet rule due to the presence of vacant d-orbitals. So they form multiple bonds. The stability depends on the repulsion forces between the ions which is governed by size of the atoms.

Complete step by step solution:
-The periodic table is divided in 4 different blocks namely s-block, p-block, d-block and f-block. They show different variations with respect to the change in the oxidation states of the atoms as the electrons present in the valence shell are different in different blocks.
-The different blocks are classified on the basis of the location of the last valence electron entering the shell of the atom. In s-block elements, the valence electrons enter the s-subshell and in p-block, the valence electrons enter the p-subshell. In d-block, the valence electron enters the d-subshell due to the Aufbau’s principle.
- The configuration of p-block elements can be shown as $n{{s}^{2}}n{{p}^{1-6}}$. The oxidation state of p-block elements varies by 2 units. This is because the electrons are excited from the s-subshell in p-block elements creating the number of unpaired electrons as 2.
-All the given central atoms belong to group 14 of the periodic table and have vacant d-orbitals and so do not have the covalency of only 4 unlike carbon which does not have d-orbitals. So, they can show covalencies of both +4 and +6.
-Due to the covalency of +6, the central atoms can link themselves with 6 radicals or 6 negative ions. But the stability of the compound depends on the size of both the surrounding atom as well as the central atom.
-The smaller central atom can bond with smaller surrounding atoms because the larger atoms will repel more leading to instability. The larger central atom can bond with the larger surrounding atoms as well due to the reduced repulsion forces owing to the size.
-The size of silicon is smallest among the given central atoms and so it can bond with the smaller surrounding atoms which have the size similar to that of silicon. So it can bond with 6 F atoms but not with 6 Cl atoms.
-The chlorine atoms are larger in size and each atom has 3 lone pairs which repel with each other. Size of silicon is small and so it becomes difficult for it to make bonds with 6 Cl atoms as the lone pairs are not able to be separated from each other.

Therefore, the least stable of all the ions formed is ${{\left[ SiC{{l}_{6}} \right]}^{-2}}$ and the correct option is D.

Note: The –OH surrounding molecule is larger than the –F and –Cl surrounding atom. But it has only 2 lone pairs while –Cl has 3 lone pairs which lead to more repulsion. Also, -OH is attached to Sn which is larger in size than Si and so the compound is more stable.