In ligand field theory, the nature of ligands plays a crucial role in the energy splitting of d-orbitals associated with metal ions. Strong field ligands induce a greater splitting of d-orbitals compared to weak field ligands.

Strong field ligands are characterized by their ability to create a significant energy difference between the d-orbitals of a metal ion. This phenomenon occurs due to their strong interactions with the metal ion, resulting in increased electron repulsion within the d-orbitals. Notable examples of strong field ligands include cyanide ($\text{CN}^-$), carbon monoxide ($\text{CO}$), and ammonia ($\text{NH}_3$).

Conversely, weak field ligands result in a smaller energy difference between the d-orbitals. This is attributed to their less effective interactions with the metal ion, which leads to reduced electron repulsion in the d-orbitals. Common examples of weak field ligands are water ($\text{H}_2\text{O}$), chloride ($\text{Cl}^-$), and hydroxide ($\text{OH}^-$).

The strength of a ligand can be assessed based on its capacity to donate electrons to the metal ion, a property that is influenced by the size of its donor atom and its electronegativity. Generally, ligands with smaller donor atoms and higher electronegativity are classified as strong field ligands, while those with larger donor atoms and lower electronegativity are considered weak field ligands.

In summary, in ligand field theory, strong field ligands such as cyanide ($\text{CN}^-$), carbon monoxide ($\text{CO}$), and ammonia ($\text{NH}_3$) create a significant energy split between d-orbitals due to their robust interactions and resultant electron repulsion. In contrast, weak field ligands like water ($\text{H}_2\text{O}$), chloride ($\text{Cl}^-$), and hydroxide ($\text{OH}^-$) lead to a smaller energy split because of their weaker interactions. The overall strength of a ligand is determined by its electron-donating ability, which is closely related to its electronegativity and the size of its donor atom.