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Construct an Orgel diagram for chromium(iii) in an octahedral ligand field and indicate on it the positions of the spin-allowed bands of [cr(h2o)6]cl3, which are observed at 265, 408 and 575 nm. a. State, with reasons, whether the bands in the spectrum of CoCl42- would be ii. of higher or lower intensities than those in the [cr(h2o)6]cl3 spectrum.
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## Orgel Diagram for Cr(III) in an Octahedral Field:
1. d-orbital Splitting:
In an octahedral field, the five d-orbitals split into two sets: a lower energy set (t2g) consisting of dxy, dyz, and dxz orbitals, and a higher energy set (eg) consisting of dx2-y2 and dz2 orbitals.
The energy difference between these sets is denoted as Δo (octahedral splitting energy).
2. Orgel Diagram:
The Orgel diagram for Cr(III) (d3 configuration) shows the possible electronic transitions within the d-orbitals.
Since Cr(III) has three electrons, the ground state configuration is t2g3, with all three electrons in the lower energy t2g orbitals.
The possible transitions are:
t2g → eg (Δo)
3. Spectral Bands and Assignments:
265 nm: This corresponds to the highest energy transition, which is a spin-allowed transition from the t2g set to the eg set, represented by the Δo energy gap in the Orgel diagram.
408 nm: This band is also spin-allowed and corresponds to a different electronic transition within the d-orbitals.
575 nm: This band is also spin-allowed, likely representing a transition from the t2g ground state to an excited state within the t2g set.
4. CoCl42- vs. [Cr(H2O)6]Cl3:
Intensities: The intensities of the bands in CoCl42- will likely be higher than those in [Cr(H2O)6]Cl3.
Reasoning: The intensity of d-d transitions is influenced by the nature of the ligands.
CoCl42-: The Cl- ligand is a strong field ligand, leading to a larger Δo and more intense color. Cl- ligands also favor Laporte-allowed transitions due to their lower symmetry.
[Cr(H2O)6]Cl3: The H2O ligand is a weaker field ligand, resulting in a smaller Δo and less intense color. Water ligands are more symmetric and thus promote Laporte-forbidden transitions, leading to lower intensity bands.
Note: For a full interpretation, a more detailed analysis including spin selection rules (Hund's rules) and Laporte selection rules is necessary. However, the above explanation provides a basic understanding of the Orgel diagram and the factors affecting band intensities in transition metal complexes.
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