Question

The ligand to metal charge transfer (LMCT) transition energy of the given ions follows the order

• A. [WO$_4$]$^{2-}$ $>$ [MoO$_4$]$^{2-}$ $>$ [CrO$_4$]$^{2-}$
• B. [CrO$_4$]$^{2-}$ $>$ [MoO$_4$]$^{2-}$ $>$ [WO$_4$]$^{2-}$
• C. [MoO$_4$]$^{2-}$ $>$ [CrO$_4$]$^{2-}$ $>$ [WO$_4$]$^{2-}$
• D. [MoO$_4$]$^{2-}$ $>$ [WO$_4$]$^{2-}$ $>$ [CrO$_4$]$^{2-}$

Hint:

In LMCT, heavier transition metals (down the group) show lower transition energy due to more diffuse orbitals

Answer 1

The Correct Option is B

Step 1: Understand LMCT transition.
LMCT (Ligand to Metal Charge Transfer) involves transfer of electron density from ligand (O$^{2-}$) to the metal centerThe energy required depends on the ability of metal to accept electrons

Step 2: Identify the metal oxidation states.
All three species have metals in +6 oxidation state:
\[ \text{Cr}^{6+}, \quad \text{Mo}^{6+}, \quad \text{W}^{6+} \]

Step 3: Effect of metal size and energy levels.
Down the group (Cr → Mo → W), atomic size increases and metal orbitals become more diffuseThis lowers the energy gap between ligand orbitals and metal orbitals

Step 4: Relation with LMCT energy.
Lower energy gap $\Rightarrow$ lower LMCT transition energyThus, heavier metals require less energy for LMCT

Step 5: Apply periodic trend.
\[ \text{Cr} > \text{Mo} > \text{W} \]
Thus, LMCT energy decreases down the group

Step 6: Arrange in decreasing order.
\[ [\text{CrO}_4]^{2-} > [\text{MoO}_4]^{2-} > [\text{WO}_4]^{2-} \]

Step 7: Conclusion.
\[ \boxed{[\text{CrO}_4]^{2-} > [\text{MoO}_4]^{2-} > [\text{WO}_4]^{2-}} \]