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: Meaning of LMCT energy.
This is the energy to move an electron from oxygen to the metal, and it falls as the metal accepts electrons more easily.

Step 2: Trend down the group.
Going from chromium to molybdenum to tungsten the metal orbitals spread out and sit lower, so the charge transfer needs less energy.

Step 3: Order.
\[ [\text{CrO}_4]^{2-} > [\text{MoO}_4]^{2-} > [\text{WO}_4]^{2-} \]

Step 4: Answer.
\[ \boxed{[\text{CrO}_4]^{2-} > [\text{MoO}_4]^{2-} > [\text{WO}_4]^{2-}} \]