CBSE Class 12 Chemistry Notes Chapter 9 Coordination Compounds

Coordination compounds involve a central atom bonded to anions or neutral molecules, maintaining their identity in solution and solids. 

• Werner's theory classified metal valencies into primary (ionisable) and secondary (non-ionisable), but had limitations. 
• Isomerism types include geometrical, optical, linkage, coordination, ionisation, and solvate. 
• Magnetic properties vary between inner and outer orbital complexes. 
• Ligand and central atoms are bonded by covalent bonds.
• Crystal Field Theory assumptions include ligands as point charges. 
• Stability is measured by stability constants. 
• Drawbacks include neglect of ligand covalency. 
• Metal carbonyls exhibit σ and π bonding. 
• Coordination compounds find applications in analysis, metal extraction, purification, and biological systems like chlorophyll and hemoglobin.

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Coordination Compounds Notes

Werner’s Theory of Coordination Compounds 

• Werner's theory was proposed to determine the number of ligand-metal bonds.
• It also determines the arrangement of bonds that occur in transition metal cations.
• The theory was proposed by Alfred Werner to formulate the design of coordination compounds.

Postulates of Werner Theory

• Werner proposed two types of valencies - primary for oxidation state and ionization, and secondary for coordination number and geometry.
• Werner's theory explains complex formation and properties based on primary and secondary valencies.
• While successful in explaining certain properties, Werner's theory fails to account for color, magnetism, and geometry of coordination compounds.

Werner Theory

Coordination entity

• Consists of a central metal atom or ion.
• Surrounded by a fixed number of ions or molecules forming coordination bonds.
• Coordination entity examples include [CoCl₃(NH₃)₃] and [Fe(CN)₆]^4-

Central atom/ion

• Atom or ion located at the center of a coordination compound.
• It forms coordination bonds with surrounding ligands, defining the structure and properties of the compound.
• Examples include Ni²⁺ in [NiCl₂(H₂O)₄], Co³⁺ in [CoCl(NH₃)₅]²⁺, and Fe³⁺ in [Fe(CN)₆]^3-.

Ligands

• Ligands are ions or molecules bound to the central atom/ion in coordination entities.
• Anionic, Cationic and Neutral ligands are three types of ligands that are categorised based on the amount of charge.
• Ligands can be unidentate (Cl⁻), didentate (ethane-1,2-diamine), or polydentate (EDTA^4-), contributing to the stability of chelate complexes.

Ligands

Coordination Number

• The number of ligand donor atoms directly bonded to the central metal ion.
• It is also known as ligancy.
• Based solely on the number of sigma bonds formed by the ligands with the central metal ion; pi bonds are not considered in determining the coordination number.

Coordination Sphere

• It encloses the central atom/ion and ligands within square brackets.
• The coordination sphere represents the core structure of the coordination compound, while counter ions are written outside the bracket.

Coordination Polyhedron

• Coordination polyhedron defines the spatial arrangement of ligand atoms around the central atom/ion.
• Common shapes include octahedral, square planar, and tetrahedral, exemplified by [Co(NH₃)₆]³⁺, [Ni(CO)₄], and [PtCl₄]^2–, respectively.

Oxidation Number of Central Atom

• Represents the charge the central atom would bear if all ligands and shared electron pairs were removed.
• The oxidation number depends upon the electron donated by ligands.

• Denoted by a Roman numeral in parentheses after the coordination entity name, such as Cu(I) in [Cu(CN)₄]^3– for copper with an oxidation state of +1.

Homoleptic and Heteroleptic Complexes

• Homoleptic complexes feature a metal bound to only one type of donor group, such as [Co(NH₃)₆]³⁺.
• Heteroleptic complexes involve a metal bound to multiple types of donor groups, as seen in [Co(NH₃)₄Cl₂]⁺.

Properties of Coordination Compounds

• Coordination compounds absorb light by the use of electronic transition.
• High spin complexes or outer orbital are filled by hybridisation.
• If coordination compounds contain unpaired electrons, then they are paramagnetic in nature.
• On the other hand, if coordination compounds contain paired electrons, they are diamagnetic.
• It consists of coloured compounds as it contains a lone pair of electrons.

Nomenclature of Coordination Compounds

Formula Writing

• Central atom listed first, followed by ligands alphabetically.
• Enclosed in square brackets, with ligand formulas in parentheses.

Naming Rules

• Cation named before ligands alphabetically.
• Anionic ligands end in "-o"; neutral and cationic ligands are named similarly.
• Oxidation state indicated by Roman numeral in parenthesis.

Isomerism in Coordination Compounds 

Stereoisomerism:

• Geometrical Isomerism: Different spatial arrangements due to restricted rotation.
• Optical Isomerism: Non-superimposable mirror images due to chirality.

Structural Isomerism:

• Linkage Isomerism: Different ligands coordinated through different donor atoms.
• Coordination Isomerism: Exchange of ligands between cationic and anionic entities.
• Ionisation Isomerism: Exchange of ions inside and outside the coordination sphere.
• Solvate Isomerism: Different arrangements of water molecules in the coordination entity.

Isomerism

Bonding in Coordination Compounds

• Coordination Bonds: Formed between the central metal atom/ion and ligands.
• Ionic Bonds: Between metal cations and anions outside the coordination sphere.
• Covalent Bonds: Sharing of electron pairs between the metal and ligands, influencing stability and reactivity.

Valence Bond Theory

• Describes the formation of coordination compounds based on the overlap of atomic orbitals between metal and ligands.
• Accounts for formation, magnetic behavior, and geometric shapes but lacks quantifying magnetic behavior and explaining optical properties.
• Specifies the relationship between the magnetic moment observed and the bond form. 
• Limitation: While it explains bonding and geometry, it falls short in predicting magnetic behavior and optical properties of coordination compounds.

Valence Bond Theory

Magnetic Properties of Coordination Compounds

• Magnetic moment measurements provide insights into the number of unpaired electrons in coordination compounds.
• For transition metals with up to three d electrons, available d orbitals hybridize with s and p orbitals, influencing magnetic behavior.
• Complications arise with d4 and d5 ions due to spin pairing, leading to variations in magnetic moments observed in different coordination compounds.

Crystal Field Theory

CFT explains the bonding in coordination compounds based on the electrostatic interaction between metal ions and ligands.

• It describes how the d orbitals of the central metal ion split in energy levels in the presence of ligand fields.
• CFT provides insights into the spectral and magnetic properties of coordination compounds based on the arrangement of d orbitals in the crystal field.

Crystal Field Theory Limitations

• CFT does not consider the covalent nature of metal-ligand bonds, which can influence the splitting of d orbitals.
• The theory assumes ligands as point charges, oversimplifying the complex nature of ligand-metal interactions.
• While useful for explaining magnetic and spectral properties, CFT has limitations in predicting certain aspects of coordination compounds, such as color and reactivity.

Crystal Field Theory

Color in Coordination Compounds

• The absorption of light in coordination compounds occurs due to d-d electronic transitions within the metal ion's d orbitals.
• The color of coordination compounds is influenced by the arrangement of ligands around the central metal ion, as explained by Ligand Field Theory.
• The color of a coordination compound often depends on the identity of the central metal ion and the nature of the ligands surrounding it.

Bonding in Metal Carbonyls

• Synergic Bonding: Metal carbonyls exhibit a unique synergic bonding where the metal-carbon bond possesses both σ and π character.
• σ Bond Formation: The ligand-to-metal bond in metal carbonyls is primarily a σ bond.
• π Bond Formation: The metal-to-ligand bond in metal carbonyls contributes to the overall stability and unique properties of these compounds, incorporating π bonding.

Bonding in Metal Carbonyls

Applications of Coordination Compounds

• Coordination compounds play vital roles in biological systems, such as chlorophyll in photosynthesis, hemoglobin in oxygen transport, and vitamin B12 in preventing anemia.
• Many coordination compounds serve as catalysts in industrial processes, such as the Wilkinson catalyst for alkene hydrogenation.
• Complexes like [Ag(CN)₂]⁻ and [Au(CN)₂]⁻ are used for smoother and more uniform electroplating of metals like silver and gold.
• Chelate therapy uses coordination compounds to treat metal poisoning, while compounds like cisplatin inhibit tumor growth.
• Coordination compounds are employed in quantitative and qualitative chemical analysis, aiding in the identification and characterization of various substances.

There are Some important List Of Top Chemistry Questions On Coordination Compounds Asked In CBSE CLASS XII