Coordination compounds, also known as complex compounds, are an important class of compounds in chemistry that consist of a central metal atom or ion bonded to a surrounding array of molecules or anions called ligands. This topic is extensively covered in Class 12 NCERT chemistry syllabus and is crucial for understanding the behavior of metal ions in biological systems, industrial catalysts, and various materials.
1. Introduction to Coordination Compounds
Coordination compounds are formed by the combination of a central metal atom or ion and a number of surrounding ligands. These ligands can be neutral molecules (like H₂O, NH₃) or anions (like Cl⁻, CN⁻). The central metal atom/ion and ligands together form a coordination entity.
Examples:
Cu(NH₃)₄]²⁺: Copper(II) ion with four ammonia molecules.
K₄[Fe(CN)₆]: Potassium hexacyanoferrate(II).
2. Terminology in Coordination Chemistry
Coordination Sphere: The central metal ion and the ligands directly attached to it.
Coordination Number: The number of ligand donor atoms attached to the central metal ion.
Ligands: Molecules or ions that donate at least one pair of electrons to the central atom/ion.
Chelate: A complex in which a ligand forms more than one bond to the central atom/ion.
3. Types of Ligands
Ligands can be classified based on the number of donor atoms they possess:
Monodentate Ligands: Ligands that donate only one pair of electrons to the central metal atom (e.g., NH₃, H₂O).
Bidentate Ligands: Ligands that donate two pairs of electrons from two donor atoms (e.g., ethylenediamine).
Polydentate Ligands: Ligands that donate more than two pairs of electrons from multiple donor atoms (e.g., EDTA).
4. Coordination Number and Geometry
The coordination number of a complex determines its geometry:
Coordination Number 2: Linear geometry (e.g., [Ag(NH₃)₂]⁺).
Coordination Number 4: Tetrahedral (e.g., Ni(CO)₄ or square planar (e.g., Pt(NH₃)₂Cl₂]).
Coordination Number 6: Octahedral geometry (e.g., [Fe(CN)₆]³⁻).
5. Nomenclature of Coordination Compounds
Naming coordination compounds involves a systematic approach:
1. Name the cation first if it is an ionic complex.
2. Name the ligands in alphabetical order. Prefixes like di-, tri-, tetra- indicate the number of each type of ligand.
3. Name the central metal with its oxidation state in Roman numerals in parentheses.
4. For anionic complexes, the metal name ends in -ate.
Examples:
[Cr(H₂O)₆]Cl₃: Hexaaquachromium(III) chloride.
K₄[Fe(CN)₆]: Potassium hexacyanoferrate(II).
6. Isomerism in Coordination Compounds
Isomerism is common in coordination compounds and includes:
Structural Isomerism: Different bonds or connectivity (e.g., linkage isomerism, coordination isomerism).
Stereoisomerism: Same connectivity but different spatial arrangement (e.g., geometric isomerism, optical isomerism).
Read Also: Electrochemistry Class 12 Chemistry
7. Bonding in Coordination Compounds
Werner's Theory of Coordination Compounds
Werner's Theory was proposed by Alfred Werner in 1893 to explain the structures and bonding of coordination compounds. His key postulates are:
1. Primary Valency: Corresponds to the oxidation state of the central metal ion and is satisfied by negative ions (anions).
2. Secondary Valency: Corresponds to the coordination number and is satisfied by ligands. These are non-ionizable and dictate the spatial arrangement around the metal.
Experimental Evidence Supporting Werner's Theory
Primary Valency: Ionizable and satisfies the oxidation state.
Secondary Valency: Non-ionizable, indicating the number of ligands directly bonded to the central metal atom.
8. Crystal Field Theory (CFT)
Crystal Field Theory explains the electronic structure of coordination compounds by considering the effect of ligands as point charges around the central metal ion. It primarily focuses on the splitting of d-orbitals in different geometries:
Octahedral Field: The d-orbitals split into two sets - t₂g (lower energy) and eg (higher energy).
Tetrahedral Field: The splitting is the opposite of octahedral - e (lower energy) and t₂ (higher energy).
Square Planar Field: Significant splitting resulting in different energy levels for dx²-y², dxy, dz², and dxz, dyz orbitals.
9. Applications of Coordination Compounds
Coordination compounds have numerous applications:
Biological Systems: Hemoglobin (iron coordination complex), chlorophyll (magnesium coordination complex).
Industrial Catalysts: Wilkinson's catalyst for hydrogenation.
Medicinal Chemistry: Cisplatin for cancer treatment.
Analytical Chemistry: EDTA in complexometric titrations for water hardness analysis.
Summary
Coordination compounds are a vital part of chemistry with complex structures and significant applications. Understanding their nomenclature, bonding theories, isomerism, and applications provides a comprehensive insight into their behavior and uses in various fields. Werner’s Theory laid the foundation for modern coordination chemistry, explaining the spatial arrangement and bonding of these fascinating compounds. Crystal Field Theory further enhances our understanding by explaining the electronic structures and properties of coordination compounds, which are essential in fields ranging from biology to industrial catalysis.