Question

A coordination compound of iron is P that has the coordination number of iron 6 and oxidation number of iron is +3. If compound P shows geometrical as well as optical isomerism. 1 mol of compound P form 4 mole of ions when dissolved in water. The compound P may be?
A.${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]C}}{{\text{l}}_{\text{3}}}$
B.${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]C}}{{\text{l}}_{\text{2}}}$
C.${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(N}}{{\text{H}}_{\text{3}}}{\text{)]C}}{{\text{l}}_{\text{3}}}$
D.${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]C}}{{\text{l}}_{\text{3}}}$

Answer 1

Test the given conditions one by one and eliminate the wrong option, you’ll ultimately reach the correct answer. For example first check the coordination number in all the compounds and then oxidation number and so on and eliminate the option that does not fit into criteria

Complete step by step answer:
Let us check the given property one by one:
Coordination no. is the number of ligands directly attached with metal ion in the coordination sphere. It is given in the question that coordination number of iron should be 6.In All the given options has 6 coordination number if Fe. Let us move to another property.
Oxidation state: let us calculate the oxidation state in each case
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]C}}{{\text{l}}_{\text{3}}}$
Separating the ${\text{C}}{{\text{l}}^{\text{ - }}}$ ion will give the coordination sphere a charge of +3.
${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]}}^{{\text{ + 3}}}}$.
Water and en both are neutral ligands and hence charge is zero for both of them
Oxidation state of Fe ${{ + 0 \times 2 + 0 \times 2 = + 3}}$ (total charge)
Oxidation state of Fe = +3
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]C}}{{\text{l}}_{\text{2}}}$, Similarly
${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]}}^{{\text{ + 2}}}}$
Oxidation state of Fe ${{ + 0 \times 4 + 0 \times 1 = + 2}}$
Oxidation state of Fe = +2
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(N}}{{\text{H}}_{\text{3}}}{\text{)]C}}{{\text{l}}_{\text{3}}}$, Similarly
${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(N}}{{\text{H}}_{\text{3}}}{\text{)]}}^{ + 3}}$ammonia is also a neutral ligand.
Oxidation state of Fe ${{ + 0 \times 4 + 0 \times 2 = + 3}}$
Oxidation state of Fe = +3
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]C}}{{\text{l}}_{\text{3}}}$, Similarly
${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]}}^{ + 3}}$en is also a neutral ligand.
Oxidation state of Fe ${{ + 0 \times 4 + 0 \times 1 = + 3}}$
Oxidation state of Fe = +3

Since the oxidation state in B option is +2 which contradicts the given statement in question hence we’ll eliminate option B.
Now coming to next property
Any complex when added in water breaks into coordination spheres and counter ions. The coordination sphere remain intact and does not dissociate further for example let any complex , X is an monovalent ion
$[AB]X \to {[AB]^ + } + {X^ - }$
Where ${{\text{[AB]}}^{\text{ + }}}$is coordination sphere and ${{\text{X}}^{\text{ - }}}$ is counter ion. Similarly all the complex will be dissociated as:
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]C}}{{\text{l}}_{\text{3}}}$ $\to$ ${\text{ + C}}{{\text{l}}^{\text{ - }}}$ ${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]}}^{{\text{ + 3}}}}$ ${\text{ + 3C}}{{\text{l}}^{{\text{ - }}}}$
Total no. of moles of ions = 4
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(N}}{{\text{H}}_{\text{3}}}{\text{)]C}}{{\text{l}}_{\text{3}}}$ $\to$ ${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(N}}{{\text{H}}_{\text{3}}}{\text{)]}}^{ + 3}}$ ${\text{ + 3C}}{{\text{l}}^{{\text{ - }}}}$
Total no. of moles of ions = 4
${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]C}}{{\text{l}}_{\text{3}}}$ $\to$ ${{\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{4}}}{\text{(en)]}}^{ + 3}}$ ${\text{ + 3C}}{{\text{l}}^{{\text{ - }}}}$
Since all give 4 moles of ion when dissolved in water. we cannot distinguish it from this property.
Now we’ll check optical and geometrical isomers. Geometrical isomer exists when same ligands are placed at ${90^0}$and ${180^0}$Both A and C shows geometrical isomer because both of them have two different ligand and can be placed to ${90^0}$and ${180^0}$to each other in octahedral geometry. But D does not show geometrical isomers so we will eliminate option D here. And a molecule or complex is said to be optically inactive when either POS(Plane of symmetry) or COS(center of symmetry ) is present in a molecule and when both of them are active then the molecule is said to have optically active.The complex in option C has no optical isomerism. So the only correct option we have is Option A
Hence the compound P is ${\text{[Fe(}}{{\text{H}}_{\text{2}}}{\text{O}}{{\text{)}}_{\text{2}}}{{\text{(en)}}_{\text{2}}}{\text{]C}}{{\text{l}}_{\text{3}}}$
So, the correct answer is Option A.

Note:
In these types of questions we have to approach systematically, eliminating the options based on the properties or conditions given in the question. Also not all the properties given to us will help in eliminating the options but they must not be ignored, because they will ultimately direct us to the correct option.