Questions: Spectrochemical Series

Question 1

Which of the following correctly places the ligands in order of increasing field strength?

• A. CO < CN⁻ < NH₃ < H₂O < Cl⁻ < I⁻
• B. I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < CN⁻ < CO
• C. F⁻ < Cl⁻ < Br⁻ < I⁻ < H₂O < NH₃ < CN⁻ < CO
• D. I⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < CO < CN⁻

Answer: (B)

The spectrochemical series from weakest to strongest field is: I⁻ < Br⁻ < Cl⁻ < F⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO. Option A reverses the entire order. Option C reverses the halides (larger halides are weaker field, not stronger). Option D swaps CN⁻ and CO — while both are strong-field, CO is generally placed at the very end. Note that among the halides, the order follows inverse size: the larger, more polarizable halides produce weaker fields.

Question 2

A chemist observes that [Co(NH₃)₆]³⁺ absorbs at shorter wavelengths than [Co(H₂O)₆]³⁺. This is consistent with NH₃ being a stronger-field ligand than H₂O.

• T. True
• F. False

Answer: True

Absorption wavelength is inversely related to energy: shorter wavelengths correspond to higher-energy transitions. In an octahedral complex, the primary d-d absorption corresponds to promoting an electron across the Δ_oct gap. A stronger-field ligand produces a larger Δ_oct, requiring higher-energy (shorter-wavelength) light for this transition. Since [Co(NH₃)₆]³⁺ absorbs at shorter wavelengths, its Δ_oct must be larger, confirming NH₃ is a stronger-field ligand than H₂O — exactly as the spectrochemical series predicts.

Question 3

The spectrochemical series can be fully explained by the electrostatic model of crystal field theory, where ligand field strength correlates directly with ligand charge.

• T. True
• F. False

Answer: False

If field strength were purely electrostatic, anionic ligands (Cl⁻, CN⁻) should produce stronger fields than neutral ligands (NH₃, CO) simply because they carry more charge. But the series shows neutral CO and NH₃ are stronger-field ligands than anionic F⁻ and Cl⁻. The spectrochemical series actually reflects covalent interactions — particularly pi-bonding effects. Strong-field ligands like CO and CN⁻ are pi-acceptors that withdraw electron density from the metal t₂g orbitals, effectively increasing Δ. Weak-field ligands like halides are pi-donors that push electron density into the metal t₂g orbitals, decreasing Δ. This is why ligand field theory (which includes covalency) succeeds where pure CFT fails.

Question 4

Explain why the spectrochemical series places the halides in the order I⁻ < Br⁻ < Cl⁻ < F⁻, and why all four are weaker-field than neutral NH₃, despite being negatively charged.

Model answer: Among halides, larger ions are more polarizable and have more diffuse lone pairs that interact poorly with metal d-orbitals. They are also better pi-donors, which raises the t₂g energy and decreases Δ. F⁻ is the strongest-field halide because its small size and low polarizability produce the least pi-donation. But even F⁻ is weaker than neutral NH₃ because all halides are pi-donors (they have filled p-orbitals that overlap with metal t₂g orbitals and raise their energy), while NH₃ is a pure sigma-donor with no pi-donating or pi-accepting capability. The pi-donation from halides partially cancels the sigma-donation effect, reducing Δ below what NH₃ achieves through sigma bonding alone.

This is a key insight: charge alone does not determine field strength. The nature of the metal-ligand interaction — whether the ligand donates or accepts pi-electron density — is the dominant factor. This is the point where crystal field theory's electrostatic model breaks down and ligand field theory becomes necessary.