Which of the following characteristics enhances the likelihood of MLCT occurring in a complex?

The likelihood of Metal-to-Ligand Charge Transfer (MLCT) occurring in a complex is significantly enhanced by the presence of low-lying pi star orbitals. This is because MLCT involves the transition of an electron from a metal d-orbital to an empty ligand-based pi star orbital. For this transition to occur efficiently, the energy level of the pi star orbitals must be lower, making it more accessible for the electron to move from the metal center to the ligand. Therefore, when low-lying pi star orbitals are present, it means that the energy gap is smaller, and the electronic transition can take place more readily, facilitating MLCT processes.

Other factors, such as spin state, ligand field strength, and the d electron count, can play roles in different types of transitions or affect the overall stability and geometry of the complex, but the direct enhancement of MLCT is primarily linked to the energy alignment between the metal d orbitals and the ligand pi star orbitals. Thus, a complex that has low-lying pi star orbitals shows a greater propensity for MLCT transitions.