Question
The intense red color of $[Fe(bpy)_3]^{2+}$ (bpy = 2,2'-bipyridine) is due to
The correct answer is
metal-to-ligand charge transfer (MLCT)
Identifying the Cause of $[Fe(bpy)_3]^{2+}$ Color
The characteristic intense red color of the complex ion $[Fe(bpy)_3]^{2+}$ arises from specific electronic transitions within the molecule. Let's analyze the options:
Understanding Electronic Transitions
- Metal-to-Ligand Charge Transfer (MLCT): This transition involves an electron moving from a filled or partially filled metal d-orbital to an empty ligand $\pi^*$ orbital. These transitions often involve a significant change in electron distribution and can lead to very intense absorptions, resulting in vivid colors.
- Ligand-to-Metal Charge Transfer (LMCT): This involves an electron moving from a ligand orbital to an empty metal orbital. It typically requires the metal to be in a high oxidation state and the ligand to be easily oxidized.
- d-d Transition: This transition occurs when an electron moves between d-orbitals of the same metal ion. While common in transition metal complexes, d-d transitions are often less intense compared to charge transfer transitions, unless symmetry-forbidden transitions become allowed through vibronic coupling.
- Inter-Valence Charge Transfer (IVCT): This occurs in compounds containing the same metal in different oxidation states, where an electron transfers between metal centers.
Reasoning for $[Fe(bpy)_3]^{2+}$ Color
In the $[Fe(bpy)_3]^{2+}$ complex:
- The metal ion is Iron in the +2 oxidation state ($Fe^{2+}$), which has a $d^6$ configuration.
- The ligand, 2,2'-bipyridine (bpy), is a relatively good $\pi$-acceptor ligand. It possesses low-lying empty $\pi^*$ orbitals.
- The $Fe^{2+}$ ion has electrons in its d-orbitals, and the bpy ligand has accessible $\pi^*$ orbitals.
- An electronic transition can occur from the Fe(II) d-orbitals to the empty $\pi^*$ orbitals of the bpy ligand. This is a metal-to-ligand charge transfer (MLCT) process.
- MLCT transitions are known to be very intense, explaining the vivid red color of the $[Fe(bpy)_3]^{2+}$ complex. While d-d transitions are possible for $Fe^{2+}$, they typically do not account for the *intense* coloration observed. LMCT and IVCT are not relevant mechanisms in this specific complex.
Therefore, the intense red color is primarily attributed to MLCT.
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Important Questions from Electronic Spectra
- The UV-visible spectrum of $[Ni(en)_3]^{2+}$ (en = ethylenediamine) shows absorbance maxima at $11200 \text{ cm}^{-1}$, $18350 \text{ cm}^{-1}$, and $29000 \text{ cm}^{-1}$.
Absorbance maximum Electronic transition (a) $11200 \text{ cm}^{-1}$ (i) $^3A_{2g} \to ^3T_{1g} (F)$ (b) $18350 \text{ cm}^{-1}$ (ii) $^3A_{2g}\to^3T_{2g}$ (c) $29000 \text{ cm}^{-1}$ (iii) $^3A_{2g}\to^3T_{1g} (P)$
[Given: Atomic number of Ni = 28]
The correct match(es) between absorbance maximum and electronic transition is/are - In aqueous solution of $K_4[Fe(CN)_6]$, the allowed transition(s) is (are)
- The $VO_4^{3-}$, $CrO_4^{2-}$ and $MnO_4^-$ ions exhibit intense ligand to metal charge transfer transition. The wavelengths of this transition follow the order
- The lowest energy d $\rightarrow$ d transition of the complexes follow the order
In the first row high-spin transition metal complexes $[M(H_2O)_6]Cl_2$ with $d^5$ and $d^7$ metal ions, the $d-d$ transitions are
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