Overview
This lecture introduces crystal field theory as a model to explain the spectroscopic and magnetic properties of coordination compounds, focusing on d-orbital splitting, electron configurations, and how these relate to color and magnetism in transition metal complexes.
Crystal Field Theory (CFT)
- CFT explains behaviors of transition metal complexes not described by valence bond theory, especially colors and magnetic properties.
- CFT models ligands and metals as point charges, focusing on electrostatic interactions and ignoring covalent character.
- In an uncomplexed metal ion, all five d orbitals are degenerate (same energy).
- In octahedral complexes, ligands approach along axes, splitting d orbitals into higher-energy eg and lower-energy t2g sets.
- Eg orbitals (dz², dx²-y²) point directly at ligands and are higher in energy; t2g orbitals (dxy, dxz, dyz) point between ligands.
- The energy difference between eg and t2g is the crystal field splitting energy (Δoct).
Factors Affecting Crystal Field Splitting
- Δoct depends on the metal's identity, charge, d orbital shell, and the type of ligands.
- Ligands are ranked in the spectrochemical series from weak- to strong-field: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻.
- Strong-field ligands (e.g., CN⁻) cause larger splitting, leading to low-spin complexes; weak-field ligands (e.g., H₂O) cause smaller splitting, leading to high-spin complexes.
Electron Configurations & Spin States
- Electrons fill orbitals to minimize energy; pairing energy (P) also affects arrangement.
- If Δoct > P, electrons pair in t2g (low spin); if Δoct < P, electrons singly occupy all orbitals first (high spin).
- Examples: [Fe(CN)₆]⁴⁻ is low-spin (all electrons paired); [Fe(H₂O)₆]²⁺ is high-spin (four unpaired electrons).
- High- vs. low-spin arrangements differ for d⁴, d⁵, d⁶, and d⁷ octahedral complexes.
Other Coordination Geometries
- Tetrahedral complexes: Ligand arrangement leads to a different splitting pattern; usually all are high-spin because Δtet is small (~4/9 Δoct).
- Square planar complexes: Removal of two ligands (z-axis) creates a unique orbital energy pattern, often favoring low-spin.
Magnetic Properties
- Unpaired electrons make complexes paramagnetic (attracted to magnetic fields), while all paired electrons result in diamagnetic substances (repelled).
- The number of unpaired electrons determines the size of the magnetic moment, verifiable by experimental measurement.
Color in Coordination Compounds
- Color arises when d-d transitions absorb visible light; observed color is complementary to the absorbed wavelength.
- Changing ligands or metal oxidation states alters Δoct, shifting which wavelengths are absorbed (and thus the color).
- d¹⁰ complexes (e.g., Cu⁺) are often colorless; partially filled d orbitals (e.g., Cu²⁺) yield colored compounds.
Key Terms & Definitions
- Crystal Field Theory (CFT) — model explaining properties of metal complexes by electrostatic ligand-metal interactions.
- Crystal field splitting (Δoct) — energy gap between eg and t2g d orbitals in an octahedral field.
- Spectrochemical series — order of ligands by their field strength effects.
- High-spin complex — maximum unpaired electrons, occurs with weak-field ligands.
- Low-spin complex — minimum unpaired electrons, occurs with strong-field ligands.
- Pairing energy (P) — energy required to pair two electrons in one orbital.
- Paramagnetic — substance with unpaired electrons, attracted to magnetic fields.
- Diamagnetic — substance with all paired electrons, slightly repelled by magnetic fields.
Action Items / Next Steps
- Practice predicting high/low-spin configurations with different ligands and d electron counts.
- Review and apply the spectrochemical series to solve color and magnetism problems.
- Complete any assigned problems on electron configurations of transition metal complexes.