NEET Notes: Coordination Chemistry

Introduction

Coordination chemistry deals with complex compounds formed by the combination of a central metal atom or ion with surrounding ligands. These coordination compounds play a vital role in biological systems, industrial applications, and analytical chemistry. Understanding their structures, bonding, and reactions is essential for NEET aspirants.

Fundamentals of Coordination Compounds

1. Definition and Components

• Coordination compounds consist of a central metal atom or ion surrounded by ligands.

• These compounds have unique bonding patterns and properties different from simple ionic or covalent compounds.

2. Important Terminologies

• Central Metal Atom/Ion: The main metal species (e.g., Fe, Co, Cu).

• Ligands: Atoms, ions, or molecules that donate electron pairs to the metal center (e.g., NH₃, Cl⁻, H₂O).

• Coordination Sphere: The metal and its attached ligands enclosed in brackets [ ].

• Coordination Number: The number of ligand donor atoms directly bonded to the metal center.

• Chelation: A process where a ligand binds to the metal through multiple donor sites, forming a stable ring structure.

Types of Ligands

1. Based on Denticity

• Monodentate Ligands: Bind through a single donor atom (e.g., Cl⁻, NH₃).

• Bidentate Ligands: Bind through two donor atoms (e.g., ethylenediamine - en).

• Polydentate Ligands: Bind through multiple donor atoms (e.g., EDTA, which has six donor sites).

2. Based on Charge

• Neutral Ligands: No charge (e.g., NH₃, CO, H₂O).

• Anionic Ligands: Carry a negative charge (e.g., Cl⁻, CN⁻, NO₂⁻).

• Cationic Ligands: Rare but exist in some special cases.

Coordination Number and Geometry

1. Common Coordination Numbers and Their Geometries

• 2 (Linear): [Ag(NH₃)₂]⁺

• 4 (Tetrahedral or Square Planar): [Ni(CO)₄] (tetrahedral), [PtCl₄]²⁻ (square planar)

• 6 (Octahedral): [Fe(CN)₆]³⁻, [Co(NH₃)₆]³⁺

2. Factors Affecting Coordination Geometry

• Size and charge of metal ion

• Nature and size of ligands

• Electronic configuration of the metal

Bonding in Coordination Compounds

1. Werner’s Theory

• Proposed by Alfred Werner, it explains primary and secondary valencies in coordination compounds.

• Primary Valency: Satisfied by anions (oxidation state of metal).

• Secondary Valency: Satisfied by ligands (coordination number).

2. Valence Bond Theory (VBT)

• Explains bonding based on hybridization of metal orbitals.

• Weak field ligands (e.g., Cl⁻, H₂O) → High-spin complexes.

• Strong field ligands (e.g., CN⁻, CO) → Low-spin complexes.

3. Crystal Field Theory (CFT)

• Explains the splitting of d-orbitals in the presence of ligands.

• Octahedral Complex: Splits into t₂g (lower energy) and eg (higher energy) orbitals.

• Tetrahedral Complex: Splits into eg (lower energy) and t₂g (higher energy) orbitals.

• Spectrochemical Series: CN⁻ > NO₂⁻ > en > NH₃ > H₂O > OH⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻ (increasing ligand field strength).

Isomerism in Coordination Compounds

1. Structural Isomerism

• Ionization Isomerism: Exchange of counter ions and ligands (e.g., [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br).

• Hydrate Isomerism: Different positions of water molecules.

• Linkage Isomerism: Ligand can bind through different atoms (e.g., NO₂⁻ as nitro or nitrito).

2. Stereoisomerism

• Geometrical Isomerism: Different spatial arrangements of ligands (e.g., cis-[Pt(NH₃)₂Cl₂] and trans-[Pt(NH₃)₂Cl₂]).

• Optical Isomerism: Mirror image isomers (chiral complexes, e.g., [Co(en)₃]³⁺).

Color and Magnetic Properties of Coordination Compounds

1. Cause of Color

• d-d transitions: Electrons absorb energy and move between split d-orbitals.

• Charge transfer transitions: Metal-to-ligand or ligand-to-metal charge transfer.

• Different ligand field strengths influence color.

2. Magnetic Properties

• Paramagnetic complexes: Have unpaired electrons.

• Diamagnetic complexes: Have all paired electrons.

• Spin states (High-spin vs. Low-spin): Depend on ligand field strength and electron pairing.

Stability of Coordination Compounds

1. Factors Affecting Stability

• Nature of Metal Ion: Higher charge, smaller size → More stable complexes.

• Nature of Ligand: Chelating ligands form more stable complexes.

• Entropy Consideration: Chelation increases entropy, making complexes stable.

2. Stability Constant (Kstab)

• A measure of how strongly a ligand binds to a metal ion.

• Larger Kstab → More stable complex.

Applications of Coordination Compounds

1. Biological Importance

• Hemoglobin (Fe complex): Oxygen transport in blood.

• Chlorophyll (Mg complex): Photosynthesis in plants.

• Vitamin B₁₂ (Co complex): Essential for metabolism.

2. Industrial Uses

• Catalysts: Wilkinson’s catalyst ([RhCl(PPh₃)₃]) for hydrogenation.

• Electroplating: [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ in silver and gold plating.

• Dyes and Pigments: Prussian blue ([Fe₄[Fe(CN)₆]₃]) used as a pigment.

3. Medicinal Applications

• Cisplatin ([PtCl₂(NH₃)₂]): Used in cancer treatment.

• EDTA (Ethylenediaminetetraacetate): Used for metal poisoning treatment.

Conclusion

Coordination chemistry is a crucial branch of inorganic chemistry that explains the structure, bonding, and reactivity of metal-ligand complexes. These compounds have diverse applications in biological systems, industry, and medicine. Understanding their properties and behavior is essential for NEET preparation and real-world applications.