Coordinate Compounds - Ligands and Types of Ligands

  • Definition of coordinate compounds
  • Introduction to ligands
    • Molecules or ions that can donate electron pairs to form a coordinate bond
  • Types of ligands
    • Monodentate ligands
      • Ligands that donate only one electron pair
    • Bidentate ligands
      • Ligands that donate two electron pairs
    • Polydentate ligands
      • Ligands that donate more than two electron pairs
  • Examples of monodentate ligands
    • H₂O (water)
    • NH₃ (ammonia)
    • Cl⁻ (chloride ion)
    • CN⁻ (cyanide ion)
  1. Examples of bidentate ligands:
  • Ethylenediamine (en)
  • Glycinato ion (gly)
  • Oxalato ion (ox)
  • Diethyldithiocarbamate (de)
  1. Examples of polydentate ligands:
  • EDTA (ethylene diamine tetraacetic acid)
  • CDTA (cyclohexane diamine tetraacetic acid)
  • DTPA (diethylenetriaminepentaacetic acid)
  1. Chelation and chelating agents:
  • Chelation: Formation of a chelate by a metal ion with a multidentate ligand
  • Chelating agents: Multidentate ligands that can form chelates
  1. Isomerism in coordination compounds:
  • Stereoisomerism:
    • Geometrical isomerism
    • Optical isomerism
  • Linkage isomerism
  • Coordination isomerism
  1. Geometrical isomerism:
  • Cis-trans isomerism
  • Caused by the presence of a bidentate or polydentate ligand
  • Examples: [CoCl₂(NH₃)₄]²⁺ (geometric isomers: cis and trans)
  1. Optical isomerism (enantiomerism):
  • Presence of chiral centers in the coordination sphere
  • Examples:
    • [Co(en)₃]³⁺ (three optical isomers)
    • [Cr(en)₂Cl₂]⁺ (two optical isomers)
  1. Linkage isomerism:
  • Different ligands are coordinated through different atoms
  • Examples: [Co(NH₃)₅(NO₂)]²⁺ and [Co(NH₃)₅(ONO)]²⁺
  1. Coordination isomerism:
  • Interchange of ligands between the cation and anion
  • Examples: [Cr(NH₃)₅(SO₄)]Br₂ and [Cr(NH₃)₅Br]SO₄
  1. Nomenclature of coordinate compounds:
  • Central metal atom/ion written first
  • Ligands named alphabetically
  • Greek prefixes used for multiple ligands of the same kind
  • Example: [Co(NH₃)₃Cl₃] → trichloridotriamminecobalt(III)
  1. Test for the presence of a coordinate bond:
  • Pale blue coloration with copper(II) ions:
    • NH₃ + Cu²⁺ → [Cu(NH₃)₄]²⁺, which is pale blue in color
  1. Color in coordination compounds:
  • Transition metal complexes often exhibit vibrant colors
  • This is due to electronic transitions within the d-orbitals of the metal ion
  • For example, the complex [Cu(H₂O)₆]²⁺ appears blue, while [Ni(H₂O)₆]²⁺ appears green
  1. Bonding in coordination compounds:
  • Coordinate bonds are formed through the donation of electron pairs from ligands to the central metal ion
  • These bonds are generally covalent in nature
  • Some compounds may exhibit ionic character if there is a large difference in electronegativity between the metal and the ligand
  1. Crystal field theory (CFT):
  • Describes the splitting of d-orbitals in an octahedral ligand field
  • Eg and t2g orbitals are split due to electrostatic interactions with the surrounding ligands
  • The energy difference between these orbitals gives rise to different colors in coordination compounds
  1. Ligand field theory (LFT):
  • A more advanced theory that considers both sigma and pi bonding interactions between the ligands and the metal ion
  • Provides a more accurate description of the bonding and electronic structure of coordination compounds
  1. Factors influencing the stability of coordination compounds:
  • Nature of the metal ion: Charge, size, and electron configuration
  • Nature of the ligand: Donor strength and size
  • Chelating ability of the ligand
  • Overall coordination number and geometry of the complex
  1. Isomerism in coordination compounds:
  • Structural isomerism:
    • Ionization isomerism
    • Hydrate isomerism
    • Coordination isomerism
  • Stereoisomerism:
    • Geometrical isomerism
    • Optical isomerism
  1. Ionization isomerism:
  • Exchange of anions between the metal and the ligand
  • Examples: [Cr(NH₃)₅Cl]Cl₂ and [CrCl(NH₃)₅]Cl₂
  1. Hydrate isomerism:
  • Exchange of water molecules with other ligands
  • Examples: CoCl₂·6H₂O and CoCl₃·3H₂O
  1. Geometrical isomerism in square planar complexes:
  • Can only occur when there are two different ligands in the complex
  • The two isomers are cis and trans
  • Examples: [Pt(NH₃)₂Cl₂] and [Pt(NH₃)Cl₂(PPh₃)]
  1. Optical isomerism in octahedral complexes:
  • Only occurs when there is a chiral ligand or a chiral metal ion present
  • Results in the formation of enantiomers
  • Examples: [Pt(NH₃)₂Cl₄] and [Pt(NH₃)₂Cl₂Br₂]