Overview
This lecture discusses how the color of transition metal complexes depends on the ligands attached, their crystal field splitting strength, and related factors such as oxidation state and period. It also covers how to use the spectrochemical series to predict color absorption and ranking of splitting energies.
Ligands and Crystal Field Splitting
- The color of a transition metal complex is affected by the type of ligand (ligan) attached.
- Ligands cause d orbitals to split into higher energy (eg) and lower energy (t2g) sets.
- Strong field ligands create a large crystal field splitting energy (Δ, delta).
- Weak field ligands produce a small crystal field splitting energy.
- The spectrochemical series arranges ligands by their field strength.
- Water (H2O) is a reference point and is considered a weak field ligand.
- Ligands such as NH3 and CN– are strong field ligands.
Predicting Splitting and Absorption (Δ and Color)
- Use the spectrochemical series to determine if a ligand causes large or small splitting.
- Large crystal field splitting requires absorption of higher energy (shorter wavelength) light (blue/violet).
- Small splitting absorbs lower energy (longer wavelength) light (red/orange).
- The actual color observed is the complement of the color absorbed.
Ranking and Comparing Complexes
- When ligands are the same, consider the oxidation number: higher oxidation state means larger splitting.
- If both ligands and oxidation number are the same, look at the period: higher periods (e.g., 4d vs 3d) have more splitting due to greater nucleus-ligand attraction and repulsion.
- Other factors like atomic distance, ligand size, and electronegativity can also affect splitting.
Worked Examples & Strategies
- To identify which complex absorbs the longest wavelength, focus on complexes with weak field ligands and similar metal centers/oxidation states.
- To rank complexes by splitting energy, compare ligand strength first, then oxidation state, then period if needed.
Key Terms & Definitions
- Ligand — A molecule or ion that binds to a central metal ion in a complex.
- Crystal Field Splitting Energy (Δ) — The energy gap between split d orbitals in a crystal field.
- Spectrochemical Series — An ordered list of ligands based on their field strength.
- Strong Field Ligand — Causes large splitting; found to the right in the spectrochemical series.
- Weak Field Ligand — Causes small splitting; found to the left in the spectrochemical series.
- Oxidation Number — Charge assigned to a metal ion in a complex.
- Period — Row in the periodic table; higher period metals (like 4d, 5d) have larger splitting than lower (3d).
Action Items / Next Steps
- Practice determining oxidation numbers for various complexes.
- Use the spectrochemical series to predict splitting and absorbed light.
- Create flashcards summarizing key concepts and examples from the lecture.