(i) The exchange of free solvent molecules such as water, NH₃, alcohol etc. in the crystal lattice with a ligand in the co-ordination entity will give different isomers. These type of isomers are called solvate isomers.
(ii) If the solvent molecule is water, then these isomers are called hydrate isomers. For example, the complex with chemical formula CrCl₃ .6H₂O has three hydrate isomers as shown below.
Complex Colour No. of chloride ions in solutions [Cr(H₂O)₄Cl₂]Cl.2H₂O dark green colour 1 [Cr(H₂O)₅Cl]Cl₂.H₂O pale green colour 2 [Cr(H₂O)₆]Cl₃ violet colour 3

These two are ionisation isomers. [Co(NH₃)₅Cl]SO₄ gives white precipitate with BaCl₂ solution, but not with AgNO₃ solution. [Co(NH₃)₅SO₄]Cl gives curdy white precipitate with AgNO₃ solution but not with BaCl₂ solution.

[Co (en)₂ Cl₂]⁺ This is an octahedral complex

\(\text {(a) }\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}:{ }_{24} \mathrm{Cr} \Rightarrow{ }_{18}[\mathrm{Ar}] 4 \mathrm{~s}^{2} 3 \mathrm{~d}^{4} \)
\({ }_{21} \mathrm{Cr}^{3+} \Rightarrow{ }_{18}[\mathrm{Ar}] 3 \mathrm{~d}^{3}\)

(i) d²sp³ hybridisation (octahedral)
(ii) It has three unpaired electrons (n = 3)
(iii) So it is paramagnetic
(iv) Magnetic moment \(\left(\mu_{\mathrm{s}}\right)=\sqrt{\mathrm{n}(\mathrm{n}+2)} \mathrm{BM}\)
\(=\sqrt{3(3+2)}=\sqrt{15}=3.87 \mathrm{BM}\)
\((b) \ \left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-:}{ }_{28} \mathrm{Ni} \Rightarrow[\mathrm{Ar}] 4 \mathrm{~s}^{2} 3 \mathrm{~d}^{8} ;{ }_{26} \mathrm{Ni}^{2+} \Rightarrow{ }_{18}[\mathrm{Ar}] 3 \mathrm{~d}^{8}\)

(i) dsp² hybridisation
(ii) Geometry - square planar
(iii) No unpaired electrons- It is Diamagnetic
(iv) Magnetic moment (μ_s) = 0.

S. No	Double salts	Co-ordination compound
1.	They usually contain two simple salt in equimolar proportions	The simple salts from which they are formed may or may not be in equimolar proportion.
2.	They exists only in the solid state. In aqueous solution they dissociate completely into ions.	They exist in the solid state as well as in aqueous solution. This is because even in solution, the complex ion does not dissociate into ions.
3.	They are ionic compounds and do not contain any co-ordinate bond.	They may or may not be ion but the complex part always contain coordinate bonds
4.	The properties of the double salts are same as those of its constituent compounds.	The properties of the coordination compounds are different for its constituent bonds.
5.	In a double salt, the metal ion show their normal valency.	In a coordinate compound the metal ion satisfies its two types of valence called primary & secondary valenices.
6.	A double salt loses its identity and dissociates into its constitute simple ions in solution.	The complex ion does not lose its identity and never dissociate to give simple ions.
	Example: FeSO4 (NH4)2 SO4.6H2O	Example: K4[Fe(CN)6), K3[Fe(SCN)6)

(i) When excess of liquid ammonia is added to an aqueous solution of copper sulphate gives tetra ammine copper (II) sulphate is formed.
CUso₄ + 4NH₃ \(\rightarrow\) [Cu(NH₃)₄]SO₄
(ii) The co-ordination entity is [Cu(NH₃)₄]²⁺