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High-spin Mn(II) and FeIII) complexes are much less intensely coloured than those of CrIII). Why are they so weakly coloured?
Accepted Answer
The color of transition metal complexes arises from d-d transitions, where electrons in lower energy d orbitals absorb light and get excited to higher energy orbitals. High-spin Mn(II) and Fe(III) complexes have weak d-d transitions because:
1. High Spin: In high-spin complexes, the d orbitals are relatively spread out, leading to smaller energy differences between them. This means that the energy required for d-d transitions is higher, falling outside the visible spectrum, resulting in weak color.
2. d-Electron Configuration: Mn(II) has a d5 configuration and Fe(III) has a d5 configuration. Both ions have a half-filled d shell, which is particularly stable due to Hund's rule. This stability makes it more difficult for electrons to undergo d-d transitions.
3. Ligand Field Strength: The ligands in high-spin complexes are typically weak field ligands, leading to smaller splitting of the d orbitals, further contributing to smaller energy differences and weak color.
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