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High-spin Mn(ii) and Fe(iii) complexes are much less intensely colored than those of Cr(iii). Why are they so weakly colored?
Accepted Answer
High-spin Mn(II) and Fe(III) complexes are weakly colored due to the following reasons:
1. d-orbital configuration: Both Mn(II) and Fe(III) ions have a high-spin d5 electronic configuration. In this configuration, all five d-orbitals are singly occupied, leading to a large crystal field splitting energy (Δ). This results in the absorption of high-energy ultraviolet light, which is outside the visible region.
2. Forbidden transitions: Transitions between d-orbitals in these complexes are often spin-forbidden. This means they violate the spin selection rule, making them less likely to occur and therefore less intense.
3. Laporte rule: d-d transitions in these complexes are Laporte forbidden, meaning they involve a change in parity. These transitions are generally weak and only occur due to slight mixing of orbitals.
In contrast, Cr(III) complexes with a d3 configuration have d-d transitions that are allowed by both spin and Laporte rules. This results in intense colors in the visible region of the electromagnetic spectrum.
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