Are you struggling to wrap your head around ligands, oxidation states, and crystal fields? You aren’t alone. Coordination Compounds Class 12 Notes are among the most searched resources because this chapter is a heavyweight in both the NCERT curriculum and competitive exams like JEE and NEET. 

This guide covers the entire NCERT syllabus, breaking down complex theories into exam-ready points. Whether you are revising for your Boards or diving into coordinate chemistry for the first time, these notes will be your roadmap to success.

 

Mastering Coordination Chemistry 

In simple terms, coordination compounds are the backbone of many biological and industrial processes. From the hemoglobin in your blood to the chlorophyll in plants, these substances consist of a central transition metal atom bonded to surrounding molecules or ions called ligands. 

For a Class 12 student, this chapter is a scoring opportunity if you master the naming rules and bonding theories. These coordination compounds class 12 notes are designed to simplify complex bonding theories and naming rules for students, ensuring you don’t lose marks on technicalities. 

 

What are Coordination Compounds? (Basic Concepts) 

Before diving deep, we must distinguish between “Double Salts” and “Coordination Compounds.” 

• Double Salts: These (like Mohr’s Salt) exist only in the solid state and dissociate completely into simple ions when dissolved in water. 

• Coordination Compounds: These retain their identity in both solid and solution forms. The complex ion does not further dissociate into its constituent metal or ligands. 

Key Terms to Remember: 

1. Coordination Entity: The central metal atom/ion bonded to a fixed number of ligands.

   Ligands: Ions or molecules that donate a pair of electrons to the metal. They can be Unidentate (one donor atom), Bidentate (two donor atoms), or Ambidentate (can bind through two different atoms, like $NO_2$ or $SCN$).

   Coordination Number: The total number of sigma bonds formed between the metal and the ligands.

   Coordination Polyhedron: The spatial arrangement of the ligands around the central atom (e.g., Octahedral, Tetrahedral, Square Planar). 

 

Werner’s Theory of Coordination Compounds 

Alfred Werner was the first to propose a correct structure for coordination compounds. His theory is the foundation of this chapter and revolves around two types of valencies: 

1. Primary Valency: This corresponds to the oxidation state of the metal. It is ionizable and satisfied by negative ions. 

1. Secondary Valency: This corresponds to the coordination number. It is non-ionizable and directional, meaning it determines the shape of the complex. 

Werner used this to explain why different cobalt-ammine complexes like $[Co(NH_3)_6]Cl_3$ and $[Co(NH_3)_5Cl]Cl_2$ reacted differently with silver nitrate. In exams, you are often asked to identify the number of ionizable $Cl^-$ ions based on this theory. 

 

Nomenclature of Coordination Compounds (IUPAC Rules) 

If you want to score high, you must master the Nomenclature of Coordination Compounds. Follow these steps: 

1. Cation First: Always name the cation before the anion. 

1. Ligands in Alphabetical Order: Name ligands first, regardless of their charge, before the metal atom. 

1. Metal Suffix: If the complex ion is an anion, the metal name ends in “-ate” (e.g., Ferrate, Plumbate). For neutral or cationic complexes, the metal name remains unchanged. 

1. Oxidation State: Mention the oxidation state of the metal in Roman numerals in parentheses. 

Examples: 

• $K_4[Fe(CN)_6]$ : Potassium hexacyanoferrate(II) 

• $[Co(NH_3)_6]Cl_3$ : Hexaamminecobalt(III) chloride 

• $[Pt(NH_3)_2Cl(NO_2)]$ : Diamminechloridonitrito-N-platinum(II) 

 

Isomerism in Coordination Compounds 

Isomerism in Coordination Compounds is a favorite topic for 3-mark questions. Isomers are compounds with the same formula but different arrangements of atoms. 

1. Structural Isomerism

• Linkage Isomerism: Occurs with ambidentate ligands (e.g., $M-SCN$ vs $M-NCS$). 

• Ionization Isomerism: Exchange of ions between the coordination sphere and ionization sphere. 

• Coordination Isomerism: Exchange of ligands between cationic and anionic entities. 

• Solvate Isomerism: Difference in the number of water molecules as ligands vs. lattice water. 

2. Stereoisomerism

• Geometrical Isomerism: Found in Coordination Number 4 (Square Planar) and 6 (Octahedral). Known as cis (adjacent) and trans (opposite). 

• Optical Isomerism: Molecules that are non-superimposable mirror images (Chiral). This is very common in octahedral complexes with bidentate ligands. 

Bonding in Coordination Compounds (VBT and CFT) 

How do these atoms stay together? We use two main theories. 

Valence Bond Theory (VBT) 

VBT focuses on hybridization. By knowing the magnetic behavior, we can predict the geometry: 

• $sp^3$: Tetrahedral 

• $dsp^2$: Square Planar 

• $d^2sp^3$ or $sp^3d^2$: Octahedral 

If a complex has unpaired electrons, it is Paramagnetic; otherwise, it is Diamagnetic. 

Crystal Field Theory (CFT) 

CFT treats the interaction between metal and ligands as purely electrostatic. It explains the “splitting” of d-orbitals. 

• Octahedral Splitting: $d$-orbitals split into $t_{2g}$ (lower energy) and $e_g$ (higher energy) levels. 

• Color: The absorption of light leads to d-d transitions, which is why transition metal complexes are so colorful! 

 

Coordination Compounds NCERT Solutions (Important Questions) 

Practicing Coordination Compounds NCERT Solutions is the best way to prepare for the “Why” type questions. 

• Q: Why is $[Ni(CN)_4]^{2-}$ square planar while $[Ni(Cl)_4]^{2-}$ is tetrahedral? 

• A: $CN^-$ is a strong field ligand causing $dsp^2$ hybridization (square planar). $Cl^-$ is a weak field ligand, resulting in $sp^3$ hybridization (tetrahedral). 

• Q: What is the chelate effect? 

• A: When a di- or polydentate ligand uses two or more donor atoms to bind to a single metal ion, it forms a ring structure called a chelate. These complexes are significantly more stable than those with unidentate ligands. 

Exam-Oriented Short Notes (Quick Revision) 

• Spectrochemical Series: $I^- < Br^- < S^{2-} < SCN^- < Cl^- < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < edta^{4-} < NH_3 < en < CN^- < CO$. 

• Effective Atomic Number (EAN): $EAN = Z – \text{Oxidation State} + 2(\text{Coordination Number})$. 

• Strong Field Ligands: Pair up electrons (low spin). 

• Weak Field Ligands: Do not pair up electrons (high spin). 

FAQs – Coordination Compounds Class 12 

1. What is the difference between a double salt and a coordination compound?

A double salt loses its identity in water (breaks into ions), while a coordination compound keeps its complex structure even when dissolved. 

2. How do youdeterminethe coordination number? 

Count the total number of sigma bonds formed by ligands. Remember, bidentate ligands count as two! 

3. What are ambidentate ligands?

Ligands like $NO_2^-$ or $CN^-$ that can bind to the metal through two different atoms. 

4. Why are transition metal complexes usually colored?

Due to d-d transitions. When light falls on the complex, electrons jump from lower energy d-orbitals to higher ones, absorbing specific wavelengths. 

 

 

Conclusion: How to Score High in Coordination Chemistry 

Success in this chapter comes down to practice. Ensure you can draw Isomerism structures accurately and never skip the Nomenclature rules. The more NCERT back-exercise questions you solve, the more confident you will feel. 

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