Coordination Compounds
This section covers Coordination Compounds: nomenclature, ligands, coordination number, isomerism, Werner's ideas, valence bond theory and crystal field theory, colour, magnetism, CFSE and applications. WBCHSE questions often compare high-spin and low-spin complexes.
Core rules
- In naming, ligands are named before the metal; anionic ligands end in -o. Oxidation state of the metal is shown in Roman numerals.
- Coordination number is the number of donor atoms bonded to the central metal.
- Strong-field ligands such as CN⁻ and CO cause large splitting and low-spin complexes; weak-field ligands such as F⁻ and H₂O favour high-spin complexes.
- CFSE depends on d-electron configuration and field strength; colour arises from d–d transitions.
Worked example
Compare [Fe(CN)₆]³⁻ and [FeF₆]³⁻. Both contain Fe³⁺, a d⁵ ion. CN⁻ is a strong-field ligand, so the complex is low-spin: t₂g⁵e_g⁰, one unpaired electron, μ = √(1×3) = 1.73 BM. F⁻ is weak-field, so the complex is high-spin: t₂g³e_g², five unpaired electrons, μ = √(5×7) = 5.92 BM. The cyanide complex is also more strongly coloured because the ligand-field splitting is larger.
Traps
Do not confuse charge on the complex with oxidation state; do not count ligands instead of donor atoms; remember that geometrical isomerism needs different ligand arrangements; do not assume tetrahedral complexes are low-spin; check whether a complex is optically active.