Question

Strongest Ligand is____.
1. CN^-
2. Br^-
3. OH^-
4. F^-

Answer 1

Correct Answer - Option 1 : CN^-

The correct answer is CN^-.

• CN^- (negative) ligand is a strong field ligand because of the highest value of Δ among the given ligand.
• The strength of any ligand is determined by the amount of crystal field theory (CFT).
• ​Crystal field theory describes the net change in crystal energy resulting from the orientation of d orbitals of a transition metal cation inside a coordinating group of anions also called ligands.
• A ligand that exerts a strong crystal or ligand electrical field and generally forms low spin complexes with metal ions when possible is called a strong ligand.
• Weak field ligands induce less splitting of the crystal fields. They form complexes with high spins.
  • Examples: chloride ions, fluoride ions, etc.
• Strong field ligands result in greater splitting of the crystal field. They form complexes with low spins.
  • Examples: Co and cyanide ion.
• ​Spectrochemical series arranges ligands in order of their field strength as:
  • I– < Br– < Cl– < SCN– < F– < OH– < C2O42- < H2O < NCS– < EDTA4- < NH3 < en < CN–< CO

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• Spectrochemical series:
  • A spectrochemical series is a list of ligands ordered on ligand strength and a list of metal ions based on oxidation number, group, and identity.
  • In crystal field theory, ligands modify the difference in energy between the d orbitals (Δ) called the ligand-field splitting parameter for ligands or the crystal-field splitting parameter, which is mainly reflected in differences in color of similar metal-ligand complexes.
  • Ligands arranged on the left end of this spectrochemical series are generally regarded as weaker ligands and cannot cause forcible pairing of electrons within the 3d level, and thus form outer orbital octahedral complexes that are high spin.
  • On the other hand, ligands lying at the right end are stronger ligands and form inner orbital octahedral complexes after forcible pairing of electrons within 3d level and hence are called low spin ligands.