WBJEE 2024 Physics & Chemistry Paper

To determine which species exhibits both Ligand-to-Metal Charge Transfer (LMCT) and paramagnetism, we need to examine the electronic structure and bonding characteristics of each species.Ligand-to-Metal Charge Transfer (LMCT): This occurs when an electron is transferred from a ligand orbital (usually nonbonding or occupied $\pi$ orbitals) to an empty or partially filled metal d-orbital. For LMCT to occur, the metal should have accessible empty d-orbitals and the ligands must be capable of donating electrons.Paramagnetism: A species is paramagnetic if it has one or more unpaired electrons. This can be determined using the electronic configuration of the metal center.Let's analyze each given species: Option A: $\mathrm{MnO}_4^{2-}$In $\mathrm{MnO}_4^{2-}$, the oxidation state of Mn is +6 . The electron configuration of Mn in the +6 oxidation state is [Ar]3d^1. It has one unpaired electron, so it is paramagnetic. However, LMCT typically involves the transfer of electrons from ligands to the metal's empty d-orbitals. Since $\mathrm{MnO}_4^{2-}$ is already in a high oxidation state, it is less likely to exhibit LMCT. Option B: $\mathrm{MnO}_4^{-}$In $\mathrm{MnO}_4^{-}$, the oxidation state of Mn is +7 . The electron configuration of Mn in +7 oxidation state is $[ \mathrm{Ar}]\,3d^{0}$, which means there are no unpaired electrons. Therefore, it is diamagnetic. Additionally, it has empty d-orbitals, making it capable of exhibiting LMCT. Option C: $\mathrm{Cr}_2\mathrm{O}_7^{2-}$In $\mathrm{Cr}_2\mathrm{O}_7^{2-}$, each Cr is in the +6 oxidation state. The electron configuration of Cr in the +6 oxidation state is $[\mathrm{Ar}]\,3d^{0}$. Like $\mathrm{MnO}_4^{-}$, there are no unpaired electrons, so it is diamagnetic. However, it can exhibit LMCT due to the presence of empty d-orbitals. Option D: $\mathrm{CrO}_4^{2-}$In $\mathrm{CrO}_4^{2-}$, the oxidation state of Cr is +6 . The electron configuration of Cr in the +6 oxidation state is also [Ar] $3d^{0}$. Therefore, it is diamagnetic. Similar to $\mathrm{Cr}_2\mathrm{O}_7^{2-}$, it can exhibit LMCT due to empty d-orbitals.From the analysis, only $\mathrm{MnO}_4^{2-}$ exhibits paramagnetism (due to one unpaired electron) while also being capable of LMCT. Thus, the correct answer is:Option A: $\mathrm{MnO}_4^{2-}$.