Question: what is the difference between a weak field ligand and a strong field ligand?
Answer: Strong field and weak field ligands are semi-empirical concepts from crystal field theory. In short, weak field ligands (e.g. F-, Cl-, OH-, H2O) will result in lower splitting of the d-orbitals of the metal in the center of a complex, while strong field ligands (e.g. NH3, CN-, NO+, CO) will result in larger splitting.