Class 12 Chemistry Chapter 9: Coordination Compounds NCERT Solutions

Alfred Werner's theory, proposed in 1893, was the first to explain the structure and bonding in coordination compounds. His key postulates are:

1. Metal atoms exhibit two types of valencies: primary valency and secondary valency.
2. Primary Valency: This is usually ionizable and corresponds to the oxidation state of the metal ion. It is satisfied by negative ions. For example, in $[Co(NH\_3)\_6]Cl\_3$, the primary valency of cobalt is +3, satisfied by three chloride ions.
3. Secondary Valency: This is non-ionizable and corresponds to the coordination number of the metal ion. It is satisfied by both negative ions and neutral molecules (ligands). These ligands are directly attached to the central metal atom and project in specific directions in space, determining the geometry of the complex. In $[Co(NH\_3)\_6]Cl\_3$, the secondary valency of cobalt is 6, satisfied by six ammonia molecules.
4. The secondary valencies are directed towards the corners of a specific geometric shape (like octahedron, tetrahedron, square planar) around the central metal atom.
5. The number of primary valencies is equal to the oxidation state, and the number of secondary valencies is equal to the coordination number.

In modern terms, primary valency is the oxidation state, and secondary valency is the coordination number.

The difference in the behavior of the two mixtures lies in the nature of the compounds formed. Both are addition compounds, but one is a double salt and the other is a coordination compound.

1. Mohr's Salt (Ferrous Ammonium Sulfate): When

FeSO_4 and (NH_4)_2SO_4

are mixed in a 1:1 molar ratio, they form a double salt, ferrous ammonium sulfate,

FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O

. Double salts are ionic compounds that exist as discrete ions in the solid state and dissociate completely into their constituent ions when dissolved in water. In aqueous solution, Mohr's salt dissociates as:

FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O(aq) \longrightarrow Fe^{2+}(aq) + 2NH_4^+(aq) + 2SO_4^{2-}(aq) + 6H_2O(l)

Since

Fe^{2+}

ions are present in the solution, it gives a positive test for

Fe^{2+} . 2. Tetraamminecopper(II) Sulfate: When

CuSO_4

is mixed with aqueous ammonia in a 1:4 molar ratio, it forms a coordination compound, tetraamminecopper(II) sulfate,

[Cu(NH_3)_4]SO_4 \cdot 5H_2O

. In this compound, the copper ion is coordinated to four ammonia molecules, forming a complex ion

[Cu(NH_3)_4]^{2+}

. This complex ion is stable and does not dissociate into

Cu^{2+} and NH_3

in solution. The compound dissociates only into the complex ion and the sulfate ion:

[Cu(NH_3)_4]SO_4 \cdot 5H_2O(aq) \longrightarrow [Cu(NH_3)_4]^{2+}(aq) + SO_4^{2-}(aq) + 5H_2O(l)

Since free

Cu^{2+}

ions are not present in the solution, it does not give the characteristic test for

Cu^{2+}

ions.

Here are explanations and examples for each term:

1. Coordination Entity:

   A coordination entity consists of a central metal atom or ion bonded to a fixed number of ligands. It is often represented within square brackets and carries a net charge (positive, negative, or neutral).

   Examples:

   • Cationic complex: $[Ni(NH\_3)\_6]^\{2+\}$
   • Anionic complex: $[PtCl\_4]^\{2-\}$
   • Neutral complex: $[Ni(CO)\_4]$
2. Ligands:

   Ligands are molecules or ions that possess at least one lone pair of electrons and can donate this pair to the central metal atom or ion to form a coordinate bond. They surround the central metal atom.

   Examples:

   • Neutral ligands: $NH\_3$ (ammonia), $H\_2O$ (aqua), $CO$ (carbonyl)
   • Anionic ligands: $Cl^-$ (chloro), $CN^-$ (cyano), $OH^-$ (hydroxo)
3. Coordination Number:

   The coordination number of a central metal atom in a coordination entity is the number of ligand atoms directly attached to it. It represents the number of coordinate bonds formed by the ligands with the central metal atom.

   Examples:

   • In $[Co(NH\_3)\_4Cl\_2]^+$ , the coordination number of Co is 6 (4 from NH3 and 2 from Cl).
   • In $[Ni(CN)\_4]^\{2-\}$ , the coordination number of Ni is 4.
4. Coordination Polyhedron:

   The coordination polyhedron refers to the geometrical arrangement of the ligands around the central metal atom in a coordination entity. This arrangement is determined by the coordination number and the nature of the ligands.

   Examples:

   • An octahedral coordination polyhedron (e.g., in $[Co(NH\_3)\_6]^\{3+\}$ ).
   • A tetrahedral coordination polyhedron (e.g., in $[Zn(CN)\_4]^\{2-\}$ ).
5. Homoleptic and Heteroleptic Complexes:
   • Homoleptic Complexes: These are coordination compounds in which the central metal atom is bonded to only one type of ligand. Examples: $[Co(NH\_3)\_6]^\{3+\}$ , $[Ni(CO)\_4]$ .
   • Heteroleptic Complexes: These are coordination compounds in which the central metal atom is bonded to more than one type of ligand. Examples: $[Co(NH\_3)\_4Cl\_2]^+$ , $[Pt(NH\_3)\_2Cl\_2]$ .