CBSE Class 12 Chemistry Chapter 18: Coordination Compounds NCERT Solutions

Werner's theory, proposed by Alfred Werner, provides a foundational understanding of bonding in coordination compounds. It is based on the following postulates:

1. Metal atoms or ions exhibit two types of valencies: primary valency and secondary valency.
2. The primary valency is satisfied by negative ions and corresponds to the oxidation state of the metal ion. It is usually ionizable.
3. The secondary valency is satisfied by neutral molecules or negative ions and corresponds to the coordination number of the metal ion. These valencies are directed in space and determine the geometry of the coordination compound. Secondary valencies are generally non-ionizable.
4. The number of secondary valencies is fixed for a given metal ion.

In modern terms, primary valency is understood as the oxidation number of the central metal atom, and secondary valency is understood as the coordination number.

The difference in the behavior of the two compounds in solution is due to their classification as double salts versus coordination compounds.

When $FeSO\_4$ is mixed with $(NH\_4)\_2SO\_4$ in a 1:1 molar ratio, it forms Mohr's salt, which is a double salt. The formula is $FeSO\_4\; \backslash cdot\; (NH\_4)\_2SO\_4\; \backslash cdot\; 6H\_2O$. In aqueous solution, double salts dissociate completely into their constituent ions. Therefore, Mohr's salt dissociates into $Fe^\{2+\}$, $NH\_4^\{+\}$, and $SO\_4^\{2-\}$ ions. The presence of free $Fe^\{2+\}$ ions allows the solution to give a positive test for this ion.

On the other hand, when $CuSO\_4$ reacts with aqueous ammonia in a 1:4 molar ratio, it forms a coordination compound, tetraamminecopper(II) sulfate, with the formula $[Cu(NH\_3)\_4]SO\_4\; \backslash cdot\; 5H\_2O$. In solution, this compound does not dissociate into individual $Cu^\{2+\}$ and $NH\_3$ molecules. Instead, it dissociates into the complex ion $[Cu(NH\_3)\_4]^\{2+\}$ and $SO\_4^\{2-\}$ ions. The central $Cu^\{2+\}$ ion is strongly bonded to the ammonia ligands, forming a stable complex ion. Because the $Cu^\{2+\}$ ion is not released freely into the solution, the solution does not give the characteristic test for $Cu^\{2+\}$ ions.

In summary:

• Mohr's salt ($FeSO\_4\; \backslash cdot\; (NH\_4)\_2SO\_4\; \backslash cdot\; 6H\_2O$) is a double salt and dissociates into $Fe^\{2+\}$, $NH\_4^\{+\}$, and $SO\_4^\{2-\}$ ions.
• Tetraamminecopper(II) sulfate ($[Cu(NH\_3)\_4]SO\_4\; \backslash cdot\; 5H\_2O$) is a coordination compound and dissociates into $[Cu(NH\_3)\_4]^\{2+\}$ and $SO\_4^\{2-\}$ ions.

Here are explanations and examples for each term:

(i) Coordination entity:

A coordination entity consists of a central metal atom or ion covalently bonded to a fixed number of ligands. It is often represented within square brackets and may carry a net charge.

Examples:

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

(ii) Ligands:

Ligands are atoms, ions, or molecules 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 covalent bond. They surround the central metal atom.

Examples:

• Neutral molecules: $NH\_3$, $H\_2O$, $CO$
• Anions: $Cl^\{-\}$, $CN^\{-\}$, $OH^\{-\}$

(iii) Coordination number:

The coordination number of a central metal atom or ion in a coordination entity is the number of ligand atoms directly attached to it. It is essentially the number of coordinate bonds formed.

Examples:

• In $[Ni(NH\_3)\_6]^\{2+\}$, the coordination number of Ni is 6, as six $NH\_3$ molecules are bonded to it.
• In $[PtCl\_4]^\{2-\}$, the coordination number of Pt is 4, as four $Cl^\{-\}$ ions are bonded to it.

(iv) Coordination polyhedron:

The spatial arrangement of the ligands around the central metal atom or ion in a coordination entity is called the coordination polyhedron. The shape of the polyhedron depends on the coordination number.

Examples:

• For coordination number 6, the polyhedron is typically octahedral. Example: $[Co(NH\_3)\_6]^\{3+\}$.
• For coordination number 4, the polyhedron can be tetrahedral or square planar. Example: Tetrahedral $[Zn(CN)\_4]^\{2-\}$, Square planar $[Ni(CN)\_4]^\{2-\}$.

(v) Homoleptic:

A coordination compound is called homoleptic if the central metal ion is bonded to only one type of ligand.

Examples:

• $[Co(NH\_3)\_6]^\{3+\}$ (Central Co ion bonded only to $NH\_3$ ligands)
• $[Ni(CO)\_4]$ (Central Ni atom bonded only to $CO$ ligands)

(vi) Heteroleptic:

A coordination compound is called heteroleptic if the central metal ion is bonded to more than one type of ligand.

Examples:

• $[Co(NH\_3)\_4Cl\_2]^\{+\}$ (Central Co ion bonded to both $NH\_3$ and $Cl^\{-\}$ ligands)
• $[Pt(NH\_3)\_2Cl\_2]$ (Central Pt atom bonded to both $NH\_3$ and $Cl^\{-\}$ ligands)