What is Coordination Compounds Class 12 NCERT: Definition & Concepts

Definition and Basic Concepts of Coordination Compounds

Coordination compounds, also called coordination complexes, consist of a central metal atom or ion surrounded by molecules or ions known as ligands. These ligands donate lone pairs of electrons to the metal, forming coordinate covalent bonds.

• Central Metal Atom/Ion: Usually a transition metal with vacant orbitals.
• Ligands: Neutral molecules or anions with lone pairs (e.g., $NH_3$, $Cl^-$, $H_2O$).
• Coordination Sphere: The metal and its attached ligands enclosed in square brackets, e.g., $[Fe(CN)_6]^{4-}$.

This concept is fundamental in Class 12 NCERT Chemistry, helping students understand the structure and bonding in these compounds.

Coordination Number and Geometry Explained

The coordination number is the number of ligand donor atoms bonded directly to the central metal atom. It determines the shape of the coordination complex.

Common coordination numbers and their typical geometries:

Understanding these geometries helps in predicting the physical and chemical properties of coordination compounds.

Types of Ligands and Their Roles

Ligands are classified based on the number of donor atoms bonded to the metal:

• Monodentate Ligands: Attach through one donor atom (e.g., $Cl^-$, $NH_3$).
• Bidentate Ligands: Attach through two donor atoms (e.g., ethylenediamine $en$).
• Polydentate Ligands: Attach through multiple donor atoms (e.g., EDTA).

Ligands influence the stability, colour, and reactivity of coordination compounds. Polydentate ligands form chelates, which are more stable due to the chelate effect.

Nomenclature Rules for Coordination Compounds

Naming coordination compounds follows specific IUPAC rules:

1. Name the ligands first (in alphabetical order), followed by the central metal. 2. Use prefixes (di-, tri-, tetra-) to denote the number of identical ligands. 3. Anionic ligands end with 'o' (e.g., $chloro$, $cyano$). 4. Name the metal with its oxidation state in Roman numerals in parentheses. 5. For complex anions, the metal name ends with '-ate'.

Example: $[Co(NH_3)_5Cl]^{2+}$ is named as pentaamminechloridocobalt(III) ion.

Importance and Applications of Coordination Compounds

Coordination compounds play vital roles in various fields:

• Biological Systems: Hemoglobin and chlorophyll are coordination complexes.
• Medicine: Cisplatin is a coordination compound used in cancer treatment.
• Industrial Catalysis: Many catalysts are coordination complexes.
• Analytical Chemistry: Complex formation helps in qualitative and quantitative analysis.

Understanding these applications helps Class 12 students appreciate the relevance of coordination compounds beyond textbooks.

Worked Example: Determining Coordination Number

Problem: Find the coordination number of the central metal in $[Cr(NH_3)_4Cl_2]^+$.

Solution:

• Count the ligands attached to chromium:
• $NH_3$ ligands = 4 (each monodentate)
• $Cl^-$ ligands = 2 (each monodentate)
• Total coordination number = 4 + 2 = 6

Thus, the coordination number of chromium in this complex is 6, indicating an octahedral geometry.