What is Ligand Field Theory?
Ligand field theory explains how the d-orbitals of a central transition metal ion are split into different energy levels by the electric field created by surrounding ligands. This splitting determines color, magnetism, and reactivity of coordination compounds.
Ligand field theory combines valence bond theory with molecular orbital theory to describe how ligands electrostatically repel d-electrons, causing d-orbital splitting. The magnitude of splitting determines the color (wavelength of light absorbed) and magnetic properties of the complex.
Step-by-step worked examples
In octahedral [Fe(H₂O)₆]²⁺, which d-orbitals are higher in energy, eg or t2g?
In octahedral field geometry, ligands approach along the x, y, z axes. Orbitals pointing toward ligands (dz² and dx²−y²) repel more → higher energy (eg). Orbitals pointing between axes (dxy, dxz, dyz) repel less → lower energy (t2g). Answer: eg orbitals are higher in energy.
Why do [Fe(CN)₆]⁴⁻ (deep red) and [Fe(H₂O)₆]²⁺ (pale yellow) have different colors?
Different ligands create different crystal field splittings (Δ). CN⁻ is a strong-field ligand → large splitting (Δo is large). H₂O is a weak-field ligand → small splitting (Δo is small). The absorbed photon energy differs → different colors observed.
Predict the coordination geometry (octahedral vs tetrahedral) for [NiCl₄]²⁻ based on the d-orbital splitting.
Cl⁻ is a weak-field ligand. Weak-field ligands prefer tetrahedral geometry (smaller splitting penalty). Strong-field ligands prefer octahedral (larger splitting, more pairing). [NiCl₄]²⁻ is tetrahedral, not octahedral.
Flashcards
Quick quiz
Q1.In tetrahedral crystal field, how many d-orbital energy levels are there?
Q2.Which ligand is a strong-field ligand?
Q3.A complex absorbs blue light (high energy). What is the crystal field splitting magnitude?
Q4.Is [Fe(CN)₆]⁴⁻ high-spin or low-spin?
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Common mistakes
Assuming all transition metal complexes have octahedral geometry. — Correct: Geometry depends on the metal, ligand type, and crystal field stabilization energy.
Confusing high-spin with high energy. — Correct: High-spin has more unpaired electrons (weaker field); low-spin has fewer unpaired electrons (stronger field).
Thinking color only depends on the metal ion. — Correct: Color depends on both the metal AND the ligand's crystal field effect.
Ignoring the spectrochemical series when predicting splitting magnitude. — Correct: Different ligands produce different splitting magnitudes; use the spectrochemical series (I⁻ < Br⁻ < Cl⁻ < OH⁻ < F⁻ < H₂O < NH₃ < CN⁻).
FAQ
What is the spectrochemical series?
An order of ligands by their ability to split d-orbitals, from weak-field (I⁻) to strong-field (CN⁻, CO).
How do you determine high-spin vs low-spin?
Compare the pairing energy to the crystal field splitting (Δ). If Δ > pairing cost, low-spin; if Δ < pairing cost, high-spin.
Why do some complexes have no color?
If the d-orbital splitting energy matches no wavelength in visible light (UV or IR instead), the complex appears colorless.
Is ligand field theory the same as crystal field theory?
Ligand field theory is an extension of crystal field theory; it includes covalent character (not purely electrostatic).