What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.
The spectrochemical series is an experimentally determined series based on the absorption of light by complexes with different ligands. In this series, ligands are arranged in the order of increasing field strength as: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
Weak field ligands are those for which the crystal field splitting (Δo) is less than the pairing energy (P), i.e., Δo < P. These ligands produce small splitting of d orbitals and form high spin complexes.
Strong field ligands are those for which Δo > P. These ligands produce large splitting of d orbitals and form low spin complexes.
Explanation
The answer draws directly from the textbook context which defines spectrochemical series as an experimentally determined series based on light absorption. The key distinction between weak and strong field ligands lies in the relationship between crystal field splitting (Δo) and pairing energy (P). Weak field ligands have Δo < P resulting in high spin complexes, while strong field ligands have Δo > P resulting in low spin complexes. The series ordering from I⁻ to CO shows increasing field strength.