Question

Comment on the following statements concerning electronic spectra. (a) \(\left[\mathrm{OsCl}_{6}\right]^{3-}\) and \(\left[\mathrm{RuCl}_{6}\right]^{3-}\) exhibit LMCT bands at 282 and \(348 \mathrm{nm},\) respectively. (b) \(\left[\mathrm{Fe}(\mathrm{bpy})_{3}\right]^{2+}\) is expected to exhibit an MLCT rather than an LMCT absorption.

Short answer

(a) LMCT bands indicate ligand-to-metal electron transfer. (b) MLCT is expected due to electron donation from Fe to bpy ligand.

Step-by-step solution

Identify the Type of Transition

In electronic spectra, there are several types of electron transitions. - **LMCT (Ligand-to-Metal Charge Transfer)** involves the transfer of electrons from ligand orbitals to metal orbitals. - **MLCT (Metal-to-Ligand Charge Transfer)** involves the transfer of electrons from metal orbitals to ligand orbitals. Understanding which transition occurs depends on the energy levels of the in-volved metal and ligand orbitals.

Analyze Part (a) - LMCT in [ ext{OsCl}_6]^{3-} and [ ext{RuCl}_6]^{3-}

For ions like [ ext{OsCl}_6]^{3-} and [ ext{RuCl}_6]^{3-}, the LMCT transitions occur when ligand electrons are promoted to empty or partially-filled metal orbitals. - The LMCT bands indicate that electrons transfer from chloride ligands to the osmium and ruthenium centers. - The wavelengths 282 nm and 348 nm correspond to the specific energies required for these transitions, with shorter wavelengths indicating higher energy.

Deduce Part (b) - MLCT in [ ext{Fe}( ext{bpy})_3]^{2+}

In [ ext{Fe}( ext{bpy})_3]^{2+}, where bpy is 2,2'-bipyridine, an MLCT is expected because: - The  ext{bpy} ligands have low-energy  ext{π*} orbitals that can accept electrons. - The iron metal has filled or partially filled d orbitals which can donate electrons to these  ext{π*} orbitals, resulting in MLCT rather than LMCT.

Key concepts

LMCT transition

In electronic spectra, the Ligand-to-Metal Charge Transfer (LMCT) transition is a process where electrons are transferred from ligand orbitals to metal orbitals. This typically occurs when ligands possess high-energy lone pair electrons that can be excited to lower-energy vacant or partially filled metal orbitals.

• LMCT transitions are characterized by the promotion of electrons from ligands to the metal center, resulting in a change in the electronic distribution.
• The occurrence of an LMCT transition is highly influenced by the nature of both the ligand and metal.
• For example, in complexes like \([\text{OsCl}_6]^{3-}\) and \([\text{RuCl}_6]^{3-}\), chloride ligands with available electron pairs can transfer these electrons to the metal orbitals.

The energy required for the LMCT transition can be observed in ultraviolet-visible (UV-Vis) spectra as bands at specific wavelengths. For \([\text{OsCl}_6]^{3-}\), this is 282 nm, while for \([\text{RuCl}_6]^{3-}\), it is 348 nm. This difference in wavelength corresponds to the energies required to move the electrons and depends on how strongly the ligand electrons are held versus how easily the metal orbitals can accommodate them.

MLCT transition

A Metal-to-Ligand Charge Transfer (MLCT) transition occurs when electrons are transferred from metal orbitals to ligand orbitals. This is a common feature in complexes where the metal center has filled or partially filled d orbitals that can donate electrons to ligand orbitals, particularly those with low-energy \(\pi^*\) orbitals.

• MLCT is typically observed in coordination compounds with ligands possessing empty or low-energy \(\pi^*\) orbitals that can accept electrons.
• The energy transfer from metal to the ligand causes characteristic bands in electronic spectra.
• In the case of \[\mathrm{Fe}( ext{bpy})_3\]^{2+}, the bpy ligands with their empty \(\pi^*\) orbitals are capable of accepting electrons from the iron center.

These transitions are crucial for understanding the electronic properties and reactivity of complex ions. They also play a significant role in the development of materials for electronic and photonic applications.

ligand-to-metal charge transfer

Ligand-to-metal charge transfer is a specific type of electronic transition crucial in understanding the color and intensity of absorption in various complexes. It involves moving electrons from the orbitals of the ligand to those of the metal ion. The energies of these transitions depend on several factors:

• The orbital energies of the ligands and metal must be properly aligned for effective transfer.
• An LMCT transition often results in significant color changes due to the absorption of light at specific wavelengths.
• This transition type is common in complexes where the metal atom has high oxidation state, making it more prone to accepting electrons from ligands.

For example, in the complexes \(\left[\mathrm{OsCl}_{6}\right]^{3-}\) and \(\left[\mathrm{RuCl}_{6}\right]^{3-}\), LMCT plays a key role, with electrons moving from chloride ligands to the central metal atoms, greatly affecting their optical properties.

metal-to-ligand charge transfer

Metal-to-ligand charge transfer is a fundamental process in coordination chemistry, where electrons shift from metal centers to ligands. This process notably influences the electronic spectra of complexes. Characteristics of MLCT include:

• Occurs when a metal with abundance of electrons transfers them to ligand orbitals, often leading to visible color changes in these compounds.
• This transfer is facilitated by ligands with empty or lower-energy \(\pi^*\) orbitals, enabling electron reception from the metal.
• Complexes that successfully undergo MLCT are often integral to photovoltaic and light-emitting devices due to their unique optical properties.

In transition metal complexes like \(\left[\mathrm{Fe}( ext{bpy})_3\right]^{2+}\), metal-to-ligand charge transfer is prominent. The transfer to the \(\pi^*\) orbitals of bipyridine (bpy) ligands affects the spectroscopic outcomes, making MLCT an area of continuous fascination for chemists.