LMCT Transition Wavelengths Explained
This solution addresses the order of wavelengths for Ligand-to-Metal Charge Transfer (LMCT) transitions in the ions $VO_4^{3-}$, $CrO_4^{2-}$, and $MnO_4^-$. These transitions are responsible for the characteristic intense colors of these species.
Understanding LMCT Transitions and Wavelength
Ligand-to-Metal Charge Transfer (LMCT) involves the excitation of an electron from a ligand-based orbital (like the oxygen $2p$ orbitals) to a metal-based orbital (typically empty d-orbitals). The energy ($E$) of this electronic transition is inversely related to the wavelength ($\lambda$) of the absorbed light, as described by the equation:
$E = \frac{hc}{\lambda}$
Consequently, transitions requiring higher energy are observed at shorter wavelengths, and transitions requiring lower energy are observed at longer wavelengths.
Analyzing the Metal Oxidation States
The oxidation states of the central metal ions in the given species are:
- In $VO_4^{3-}$, Vanadium (V) is in the +5 oxidation state.
- In $CrO_4^{2-}$, Chromium (Cr) is in the +6 oxidation state.
- In $MnO_4^-$, Manganese (Mn) is in the +7 oxidation state.
Determining the Wavelength Order
For the series of d0 tetrahedral oxoanions ($VO_4^{3-}$, $CrO_4^{2-}$, $MnO_4^-$), experimental data show that the wavelength of the LMCT absorption band increases with increasing oxidation state of the central metal atom.
This observation implies that the energy required for the LMCT transition actually decreases as the metal's oxidation state increases within this specific series.
Therefore, the order of increasing wavelength is:
$VO_4^{3-} < CrO_4^{2-} < MnO_4^-$
This corresponds to the order of decreasing transition energy:
$E(VO_4^{3-}) > E(CrO_4^{2-}) > E(MnO_4^-)$
Final Order
The wavelengths of the LMCT transition follow the order $VO_4^{3-} < CrO_4^{2-} < MnO_4^-$.
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 lowest energy d $\rightarrow$ d transition of the complexes follow the order
- The intense red color of $[Fe(bpy)_3]^{2+}$ (bpy = 2,2'-bipyridine) is due to
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
