The deep purple color of potassium permanganate (KMnO₄) is primarily due to a Ligand to Metal Charge Transfer (LMCT) transition.

In KMnO₄, the central metal ion is manganese in the +7 oxidation state (Mn⁷⁺). The ligands are oxygen atoms from the permanganate ion (MnO₄^-).

An LMCT transition occurs when an electron from a molecular orbital that is primarily located on the ligand (in this case, from an oxygen atom) is excited and transferred to a molecular orbital that is primarily located on the metal (an empty d-orbital on the Mn⁷⁺ ion). This electron transfer requires energy, which is absorbed from visible light. The specific energy (and thus the specific color of light absorbed) corresponds to the purple color we observe.

Let's analyze why the other options are incorrect:

• d-d transition: This is not possible because the Mn⁷⁺ ion has a d⁰ electronic configuration. There are no d electrons to be excited to a higher energy d orbital.
• σ–σ* transition: This type of transition typically requires very high energy, corresponding to ultraviolet light, not visible light. It is not responsible for the color of KMnO₄.
• M → L charge transfer transition: A Metal to Ligand Charge Transfer (MLCT) would involve an electron moving from the metal to the ligand. For Mn⁷⁺, which is highly electron-deficient, it is much more favorable for an electron to move from the ligand to the metal (LMCT) rather than the other way around.

Final Answer: L → M charge transfer transition

Related Topics & Formulae

Charge Transfer Transitions: These are intense electronic transitions in coordination compounds where an electron moves from an orbital primarily centered on one atom to an orbital primarily centered on another. They are responsible for many of the intense colors seen in transition metal complexes. The energy required for the transition is given by the equation for the energy of a photon: $E=h\nu$ where $h$ is Planck's constant and $\nu$ is the frequency of the absorbed light.

Electronic Configurations: Understanding the d-electron count (e.g., d⁰ for Mn⁷⁺) is crucial for predicting the possibility of d-d transitions.

Oxidation States: The high oxidation state of the metal ion (Mn⁷⁺) makes it a very strong oxidizing agent and a good acceptor for electrons from ligands, facilitating LMCT.