Q9(c). [Co(bipy)₃]²⁺ and [Co(NH₃)₆]²⁺: terms, spectra and MO diagrams
For each of the compounds [Co(bipy)₃]²⁺ and [Co(NH₃)₆]²⁺, answer the following: (i) Find the ground state term symbol. (ii) Use the Tanabe–Sugano diagram to identify the predicted spectral bands. (iii) Do you expect broad or narrow absorption bands in the visible and UV regions? (iv) Sketch an MO energy level diagram for each.
Answer in brief
(i) Co²⁺ (3d⁷), high spin t_(2g)⁵e_g²: ground term ⁴T_(1g) (free-ion ⁴F). (ii) three spin-allowed bands: ν₁ ⁴T_(1g)→⁴T_(2g), ν₂ ⁴T_(1g)→⁴A_(2g), ν₃ ⁴T_(1g)→⁴T_(1g)(P). (iii) Broad d–d bands; narrow only for the weak spin-forbidden bands and the intense π→π^* bands of bipy in the UV. (iv) see the sketch.
Key points
- (i) Ground term: d⁷ high spin: t_(2g)⁵e_g², S=32 (quartet). In the field of a free ion the term is ⁴F, which splits in Oₕ into ⁴T_(1g) + ⁴T_(2g) + ⁴A_(2g); ⁴P becomes ⁴T_(1g)(P). ⁴T_(1g)(F) is the ground state. Both complexes…
- (ii) Bands (Tanabe–Sugano d⁷, high-spin side):
- (iii) Band shape: the three spin-allowed bands are broad (100–2000 cm⁻¹ width): the excited configurations (t_(2g)⁴e_g³ and t_(2g)³e_g⁴) differ from the ground one, so the Tanabe–Sugano lines have different slopes and the…
- (iv) MO diagrams: for [Co(NH₃)₆]²⁺ (pure σ-donor) the ligand σ orbitals (a_(1g),e_g,t₁ᵤ) lie below the metal d; the metal t_(2g) (non-bonding, 5 e) and e_g^* (2 e) are the frontier orbitals; Δₒ=e_g^*-t_(2g). For [Co(bipy)₃]²⁺…
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- (i) Ground term: d⁷ high spin: t_(2g)⁵e_g², S=32 (quartet). In the field of a free ion the term is ⁴F, which splits in Oₕ into ⁴T_(1g) + ⁴T_(2g) + ⁴A_(2g); ⁴P becomes ⁴T_(1g)(P). ⁴T_(1g)(F) is the ground state. Both complexes are high spin at room temperature; [Co(bipy)₃]²⁺ has the larger Δₒ and is near the spin crossover to ²E_g.
- (ii) Bands (Tanabe–Sugano d⁷, high-spin side):
| Band | Transition | [Co(NH₃)₆]²⁺ / cm⁻¹ | [Co(bipy)₃]²⁺ / cm⁻¹ | Nature |
| ν₁ | ⁴T_(1g)→⁴T_(2g) | ≈9 000 | ≈11 000 | infrared/near-IR, Δₒ related |
| ν₂ | ⁴T_(1g)→⁴A_(2g) | ≈18 000 | ≈22 000 | two-electron, weak |
| ν₃ | ⁴T_(1g)→⁴T_(1g)(P) | ≈21 000 | ≈23 000 | strongest d–d, visible |
Band positions are approximate illustrative values read from the diagram; they are not exact measurements.
- (iii) Band shape: the three spin-allowed bands are broad (100–2000 cm⁻¹ width): the excited configurations (t_(2g)⁴e_g³ and t_(2g)³e_g⁴) differ from the ground one, so the Tanabe–Sugano lines have different slopes and the vibrational (Franck–Condon) overlap spreads the band. Spin-forbidden doublet bands (²E_g, ²T_(1g)) run parallel to the ground state and are narrow but weak. In the UV, [Co(bipy)₃]²⁺ shows sharp, very intense ligand π→π^* bands (280–300 nm, ε>10⁴) and broad MLCT, while [Co(NH₃)₆]²⁺ has a broad LMCT edge.
- (iv) MO diagrams: for [Co(NH₃)₆]²⁺ (pure σ-donor) the ligand σ orbitals (a_(1g),e_g,t₁ᵤ) lie below the metal d; the metal t_(2g) (non-bonding, 5 e) and e_g^* (2 e) are the frontier orbitals; Δₒ=e_g^*-t_(2g). For [Co(bipy)₃]²⁺ the empty ligand π^* orbitals lie above the metal t_(2g); mixing stabilises t_(2g) (π back-bonding), raises Δₒ and makes MLCT t_(2g)→π^* possible.
[Co(bipy)₃]²⁺ is a mild reducing agent used in outer-sphere electron-transfer studies: the t_(2g)→ e_g change on oxidation to Co³⁺ (low spin t_(2g)⁶) makes self-exchange slow.
CoversTanabe–Sugano diagram for d⁷[Co(bipy)₃]²⁺ and [Co(NH₃)₆]²⁺terms
All parts of Question 9
- 9(a) Isotope dilution method10 marks
- 9(b) Reaction of [Mn(CO)₅]⁻ with allyl chloride10 marks
- 9(c) [Co(bipy)₃]²⁺ and [Co(NH₃)₆]²⁺: terms, spectra and MO diagrams10 marks