Electronic Spectra of High Spin Complexes

The electronic spectrum of a transition metal complex arises primarily from d-d transitions, where an electron in a lower energy d-orbital is excited to a higher energy d-orbital upon absorption of light. The characteristics of these spectra, such as the number, intensity, and sharpness of bands, depend on the metal ion, its oxidation state, the ligand field strength, and the geometry of the complex. For high spin complexes, the electron configuration in the d orbitals is determined by Hund's rule, maximizing the number of unpaired electrons.

Let's analyze each statement regarding the electronic spectra of high spin complexes:

1. Ti³⁺ complexes exhibit one sharp band.
   • Ti³⁺ has a d¹ electronic configuration.
   • In an octahedral crystal field, the d orbital splits into a lower energy t_2g set and a higher energy e_g set. The ground state configuration for a d¹ ion is \(t_{2g}^1 e_g^0\).
   • There is only one possible d-d transition: \(t_{2g}^1 \rightarrow e_g^1\). This would ideally give one absorption band.
   • However, complexes with degenerate ground or excited states (like d¹ in octahedral geometry) often undergo Jahn-Teller distortion. This distortion further splits the energy levels, leading to the splitting of the single band or causing it to appear broad rather than sharp.
   • Therefore, the statement that Ti³⁺ complexes exhibit one sharp band is generally incorrect. They exhibit one broad band, which may show splitting due to Jahn-Teller effect.
2. Co²⁺ and Cr³⁺ complexes exhibit two broad bands.
   • Co²⁺ is d⁷. In a high spin octahedral complex, the configuration is \(t_{2g}^5 e_g^2\). According to the Tanabe-Sugano diagram for d⁷, there are three spin-allowed transitions from the \(^4T_{1g}(F)\) ground state: \(^4T_{1g}(F) \rightarrow ^4T_{2g}(F)\), \(^4T_{1g}(F) \rightarrow ^4A_{2g}(F)\), and \(^4T_{1g}(F) \rightarrow ^4T_{1g}(P)\). The transition to \(^4A_{2g}(F)\) (\(\nu_2\)) often corresponds to a two-electron jump and is usually weak or not observed. Thus, Co²⁺ complexes typically show two main broad bands.
   • Cr³⁺ is d³. In an octahedral complex, the configuration is \(t_{2g}^3\). According to the Tanabe-Sugano diagram for d³, there are three spin-allowed transitions from the \(^4A_{2g}(F)\) ground state: \(^4A_{2g}(F) \rightarrow ^4T_{2g}(F)\), \(^4A_{2g}(F) \rightarrow ^4T_{1g}(F)\), and \(^4A_{2g}(F) \rightarrow ^4T_{1g}(P)\). Cr³⁺ complexes typically exhibit three broad bands.
   • Since Cr³⁺ complexes exhibit three broad bands, the combined statement that both Co²⁺ and Cr³⁺ complexes exhibit two broad bands is incorrect.
3. Mn²⁺ complexes exhibit a series of very weak and sharp bands.
   • Mn²⁺ has a d⁵ electronic configuration.
   • In a high spin octahedral complex, the configuration is \(t_{2g}^3 e_g^2\). This configuration gives rise to a high spin \(^6A_{1g}\) ground state.
   • All possible excited states arising from d-d transitions for d⁵ are of lower spin multiplicity (e.g., quartet or doublet).
   • Transitions from the sextet ground state (\(^6A_{1g}\)) to any of the quartet or doublet excited states are spin-forbidden transitions.
   • Spin-forbidden transitions are typically very weak in intensity (low molar absorptivity) and appear relatively sharp because they have minimal vibronic coupling.
   • Therefore, the statement that Mn²⁺ complexes exhibit a series of very weak and sharp bands is correct.
4. Ni²⁺ complexes exhibit three broad bands.
   • Ni²⁺ has a d⁸ electronic configuration.
   • In a high spin octahedral complex, the configuration is \(t_{2g}^6 e_g^2\). According to the Tanabe-Sugano diagram for d⁸, there are three spin-allowed transitions from the \(^3A_{2g}(F)\) ground state: \(^3A_{2g}(F) \rightarrow ^3T_{2g}(F)\) (\(\nu_1\)), \(^3A_{2g}(F) \rightarrow ^3T_{1g}(F)\) (\(\nu_2\)), and \(^3A_{2g}(F) \rightarrow ^3T_{1g}(P)\) (\(\nu_3\)).
   • These spin-allowed transitions are relatively intense and typically give rise to broad absorption bands due to vibronic coupling.
   • Therefore, the statement that Ni²⁺ complexes exhibit three broad bands is correct.

Based on the analysis, statements C and D are correct.