Match List-I with List-II:
List-I                                                             List-II
a. [Co(NH$$_3$$)$$_6$$][Cr(CN)$$_6$$]               i. Linkage isomerism
b. [Co(NH$$_3$$)$$_3$$(NO$$_2$$)$$_3$$]              ii. Solvate isomerism
c. [Cr(H$$_2$$O)$$_6$$]Cl$$_3$$                           iii. Co-ordination isomerism
d. $$cis$$-[CrCl$$_2$$(ox)$$_2$$]$$^{3-}$$              iv. Optical isomerism
Choose the correct answer from the options given below:

We need to match each coordination compound with the type of isomerism it exhibits.

(a) $$[\text{Co}(\text{NH}_3)_6][\text{Cr}(\text{CN})_6]$$ is a salt made up of two complex ions: the cation $$[\text{Co}(\text{NH}_3)_6]^{3+}$$ and the anion $$[\text{Cr}(\text{CN})_6]^{3-}$$. In coordination isomerism, the ligands can be interchanged between the two metal centres to give a different compound such as $$[\text{Cr}(\text{NH}_3)_6][\text{Co}(\text{CN})_6]$$. Since this compound contains both a cationic and an anionic complex with different metals, it exhibits coordination isomerism. So (a) matches with (iii).

(b) $$[\text{Co}(\text{NH}_3)_3(\text{NO}_2)_3]$$ contains the $$\text{NO}_2^-$$ ion, which is an ambidentate ligand. An ambidentate ligand can coordinate through more than one atom — here, $$\text{NO}_2^-$$ can bind through nitrogen (as a nitro ligand, $$-\text{NO}_2$$) or through oxygen (as a nitrito ligand, $$-\text{ONO}$$). When the same ligand coordinates through different donor atoms, the resulting isomers are called linkage isomers. So (b) matches with (i).

(c) $$[\text{Cr}(\text{H}_2\text{O})_6]\text{Cl}_3$$ is the fully hydrated form of chromium(III) chloride, where all six water molecules are in the coordination sphere and all three chlorides are in the outer sphere as counter ions. Solvate isomerism (also called hydrate isomerism when the solvent is water) arises when water molecules and anions exchange positions between the coordination sphere and the outer sphere. For instance, $$[\text{Cr}(\text{H}_2\text{O})_5\text{Cl}]\text{Cl}_2 \cdot \text{H}_2\text{O}$$ is a solvate isomer where one chloride has entered the coordination sphere while one water molecule has moved outside. So (c) matches with (ii).

(d) $$cis\text{-}[\text{CrCl}_2(\text{ox})_2]^{3-}$$ is an octahedral complex with two monodentate chloride ligands in a $$cis$$ arrangement and two bidentate oxalate ($$\text{ox}$$) ligands. The $$cis$$ isomer of this complex lacks a plane of symmetry and its mirror image is non-superimposable on itself — meaning it is chiral. This gives rise to optical isomerism, where the two mirror-image forms (called enantiomers) rotate the plane of polarised light in opposite directions. So (d) matches with (iv).

The correct matching is (a)-(iii), (b)-(i), (c)-(ii), (d)-(iv), which is Option A.