Place 0.5 ml of 0.1 M Ti2(SO4)3 into three wells of another well-plate. Add different ligands to the wells having a 0.1 M solution of Ti2+ ions (in the fume hood). Add the amounts of reagent to the corresponding well, then individually make a visual observation and determine the Amax. Complete the table below: Reactions of Titanium (II) Coordination Compounds with various ligands: ml of Reagent | Complex ion | Observed color | Amax | kJ/mol (Splitting energy) ————–|————-|—————-|——|————————- 1.5 ml of water | [Ti(H2O)]3+ | Colorless | 496 | 2414 1.5 ml of EDTA | [Ti(EDTA)] | Pastel Violet | 548 | 218 1.5 ml of [TiCl] – Pale | 4761 | 2514 | Saturated brown NaCl Question: Arrange Titanium complexes in the order of increasing and compare between this arrangement and the spectrochemical series. Explain. (Hints: number of ligands, wavelength, and the limitation of the used instrument: It can detect the wavelength between 400 nm-900 nm) Titanium complexes: EDTA+- < H2O < Cl- Spectrochemical series: Cl- < H2O < EDTA+- The arrangement is different from the spectrochemical series because…
The Correct Answer and Explanation is:
Answer:
The order of increasing ligand field strength based on the observed data (in terms of increasing splitting energy or decreasing λ_max) is:
EDTA⁴⁻ < H₂O < Cl⁻
In contrast, the spectrochemical series suggests the order:
Cl⁻ < H₂O < EDTA⁴⁻
Explanation
The spectrochemical series ranks ligands based on their ability to split the d-orbital energies of a transition metal ion in a coordination complex. Ligands that cause larger splittings are termed strong field ligands, while those causing smaller splittings are weak field ligands. According to this series, Cl⁻ is a weak field ligand, H₂O is moderate, and EDTA⁴⁻ (which binds through multiple donor atoms) is a strong field ligand. This would predict the order:
Cl⁻ < H₂O < EDTA⁴⁻, where EDTA⁴⁻ gives the highest splitting energy.
However, in the experiment, the data indicate the opposite:
- [Ti(EDTA)] shows λ_max = 548 nm → lowest Δ (218 kJ/mol)
- [Ti(H₂O)₆]³⁺ shows λ_max = 496 nm → Δ = 241 kJ/mol
- [TiClₓ] complex shows λ_max = 476 nm → highest Δ (251 kJ/mol)
This gives the order: EDTA⁴⁻ < H₂O < Cl⁻
The discrepancy likely arises from several experimental factors:
- Charge and Geometry Effects: EDTA forms a more stable, possibly distorted octahedral complex that affects the crystal field splitting differently than expected.
- Instrumental Limitations: The spectrophotometer only measures between 400–900 nm, possibly missing transitions outside this range, especially for stronger field ligands with higher Δ values and lower λ_max.
- Nature of Ligand Interaction: Though EDTA is a strong field ligand generally, its binding mode in this Ti²⁺ complex might be altered, reducing effective orbital overlap.
- Oxidation State and Coordination Number: Ti²⁺ may coordinate differently with each ligand, and Ti-EDTA may adopt a geometry or d-electron configuration that doesn’t maximize field splitting.
Thus, experimental observations diverge from theory due to both coordination chemistry subtleties and practical limitations.
