Q6(b)(i). Pale pink [Co(H₂O)₆]²+ and deep blue [CoCl₄]²-

1. Colour seen is the complement of the colour absorbed. How intense the colour is depends on ε of the band; its hue depends on where the band lies.
2. Laporte rule. In a centrosymmetric complex only transitions with a change of parity (g ↔ u) are allowed. d orbitals are g, so d–d (g → g) transitions are Laporte forbidden.
3. [Co(H₂O)₆]²+ is octahedral and has a centre of symmetry. Its bands (⁴T_(1g)(F)→⁴T_(1g)(P) for the main visible band) gain intensity only through unsymmetrical vibrations that destroy the centre for an instant (vibronic coupling). ε stays below about 10 dm³ mol⁻¹ cm⁻¹. The ion absorbs weakly in the green, so the solution is pale pink.
4. [CoCl₄]²- is tetrahedral and has no centre of symmetry, so g and u labels do not apply. The t₂ set can mix with the 4p orbitals (both t₂ in T_d), so the transition gains some d→ p (allowed) character. ε is about 500 dm³ mol⁻¹ cm⁻¹: a deep colour from a dilute solution.
5. Position. Four ligands, none pointing at the d orbitals, split them less: Δₜ≈49Δₒ. Cl⁻ is also a weaker-field ligand than H₂O in the spectrochemical series. Thus Δₒ=8400 cm⁻¹ for [Co(H₂O)₆]²+ but Δₜ=3300 cm⁻¹ for [CoCl₄]²-. The main band (⁴A₂→⁴T₁(P)) moves to lower energy, into the orange-red; the red end of the light is removed and the ion looks deep blue.
6. Both ions are high-spin d⁷ with three unpaired electrons and quartet excited states, so all these bands are spin allowed; the difference in intensity comes only from the centre of symmetry.

Application. CoCl₂ in silica gel desiccant is blue when dry (tetrahedral chloro species) and turns pink when it takes up water (octahedral aqua ion). The same change is seen when concentrated HCl is added to a pink cobalt(II) solution.