Which orbital characterizes ligands that strongly participate in MLCT transitions?

The choice indicating low lying pi star orbitals as characterizing ligands that strongly participate in metal-to-ligand charge transfer (MLCT) transitions is correct because MLCT transitions typically involve the donation of an electron from a metal's d orbital to an empty orbital of the ligand. In many cases, this is an empty pi star (π*) orbital, which is lower in energy and can efficiently accept the electron.

The presence of low lying pi star orbitals in ligands allows for effective mixing with the metal's d orbitals, facilitating transitions between them during excitation. This energy alignment is critical for the MLCT process to occur, as the energy of the absorbed light must match the energy difference between the filled d orbitals of the metal and the empty pi star orbital of the ligand.

Moreover, other options do not appropriately describe the orbitals involved in MLCT transitions. High energy d orbitals, while they can participate in some types of transitions, are typically not where electron donation occurs in a classic MLCT scenario. Unstable s orbitals generally do not have the appropriate energy levels or characteristics to participate significantly in such charge transfer processes. Filled valence p orbitals may be involved in other electronic transitions but do not play a primary role