IIT JEE Chemistry Practice Paper – Part 14 (Coordination Compounds)

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Coordination Compounds: The Architectural Marvels of Inorganic Chemistry

A Strategic Introduction for IIT-JEE Aspirants by Prof. Anil Tyagi

Future engineers and scientists, welcome. As we venture beyond simple salts and molecules, we enter the elegant and complex world of Coordination Compounds. Think of this not as another chapter, but as the study of chemical architecture where a central metal ion acts as a cornerstone, and surrounding molecules or ions, called ligands, form the intricate pillars and walls. This topic is not just crucial for IIT-JEE; it is fundamental to understanding biological systems (like haemoglobin), industrial catalysts, and modern materials. Our goal is to build a conceptual framework that is both deep and exam-ready.

1. The Core Concept: What Are Coordination Compounds?

A coordination compound is characterized by a central metal atom or ion surrounded by a set of molecules or anions known as ligands. The key differentiator from a double salt is that coordination compounds retain their identity in solution. The bond between the metal and the ligand is a special covalent bond where the ligand donates a pair of electrons to the metal. This is called a coordinate covalent bond.

• Central Metal Atom/Ion: Typically a transition metal (e.g., Fe, Co, Ni, Cu) due to their ability to possess vacant d-orbitals. It acts as a Lewis acid (electron pair acceptor).
• Ligands: Ions or molecules (e.g., NH₃, H₂O, Cl⁻, CN⁻) that donate a pair of electrons. They are Lewis bases (electron pair donors). Ligands are classified by their “denticity” (number of donor atoms):
  • Monodentate: One donor atom (e.g., NH₃, H₂O, Cl⁻).
  • Bidentate: Two donor atoms (e.g., Ethylenediamine (en), Oxalate (ox)).
  • Polydentate: Multiple donor atoms. A special and very stable class of polydentate ligands are chelates, which form ring-like structures with the metal (e.g., EDTA, which is hexadentate).

2. Fundamental Terminology: The Building Blocks

To master this chapter, you must be fluent in its language.

• Coordination Entity: The entire complex ion or molecule, e.g., [Cu(NH₃)₄]²⁺.
• Coordination Sphere: The central metal ion and its surrounding ligands, enclosed in square brackets [].
• Coordination Number (C.N.): The total number of coordinate bonds formed by the metal with the ligands. This is a critical concept for predicting geometry.
  • C.N. = 4 → Tetrahedral or Square Planar.
  • C.N. = 6 → Octahedral (most common).
• Oxidation State of the Metal: The charge it would carry if all the ligands were removed along with their electron pairs. The sum of the charges on the metal and the ligands must equal the net charge of the complex.
• IUPAC Nomenclature: A systematic method for naming complexes. The order is: Ligands in alphabetical order → Metal → Oxidation State (Roman numeral). For anions, the metal name ends with ‘-ate’ (e.g., Ferrate for Fe, Cobaltate for Co).

3. Werner’s Coordination Theory: The Historical Breakthrough

Alfred Werner’s theory (1893) was revolutionary. He proposed:

1. Metals possess two types of valencies: Primary (ionic) Valency and Secondary (non-ionic) Valency.
2. The primary valency is ionizable and corresponds to the oxidation state.
3. The secondary valency is non-ionizable and corresponds to the coordination number. It dictates the spatial arrangement (geometry) of the ligands.

4. Modern Bonding Theories: The “Why” Behind the Structure

For IIT-JEE, you must have a clear understanding of two key theories:

• Valence Bond Theory (VBT): Explains geometry based on the hybridization of the central metal atom.
  • d²sp³ hybridization (using inner d-orbitals) → Octahedral, low-spin complexes.
  • sp³d² hybridization (using outer d-orbitals) → Octahedral, high-spin complexes.
  • Limitation: VBT is qualitative and cannot explain colour or magnetic properties quantitatively.
• Crystal Field Theory (CFT): This is the most important theory for JEE. It explains colour, magnetism, and stability.
  • Core Idea: Ligands are treated as point negative charges that repel the d-electrons of the metal ion. This repulsion splits the degenerate (equal energy) d-orbitals into sets of different energies.
  • Crystal Field Splitting (Δ₀): In an octahedral field, the d-orbitals split into a higher energy e_g set (d_x²-y², d_z²) and a lower energy t_2g set (d_xy, d_yz, d_zx). The energy gap is Δ₀.
  • Applications:
    • Colour: Caused by the absorption of light energy promoting an electron from the t_2g to the e_g level (d-d transition).
    • Magnetic Properties: Depends on the number of unpaired electrons, which is determined by the magnitude of Δ₀ relative to the pairing energy (P). Small Δ₀ → High-spin complex. Large Δ₀ → Low-spin complex.
    • Spectrochemical Series: An essential series to memorize: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < CN⁻ ≈ CO. This orders ligands by their ability to cause splitting (Δ₀). Strong field ligands (right side) cause large splitting.

Professor’s Final Advice: Focus on CFT. Master the spectrochemical series. Practice numerical problems on magnetic moments and crystal field stabilization energy (CFSE). This chapter is highly scoring if your concepts are clear. Visualize the structures, understand the splitting, and you will conquer it.

– Prof. Anil Tyagi