Ligand field theory describes the electronic and magnetic properties of coordination complexes resulting from the interaction of peripheral donor atoms with 
-orbitals of the central metal atom.
Molecular orbital (MO) diagrams of -metal complexes (see above diagram for an example, and read the previous article on how it is constructed) provide the theoretical foundation for the ligand field theory. In the context of an octahedral complex, the energy separation between 
and 
, known as the ligand field splitting parameter 
, is a crucial aspect of this theory. This energy difference is responsible for distinguishing the electronic and magnetic properties among different complexes. The magnitude of for a complex with a specific metal is dependent on the type of ligands. Ligands (e.g. 
and 
) that interact strongly with the metal orbitals are called strong-field ligands, while ligands with weak interactions (e.g. 
and 
) are known as weak-field ligands.
Consider the following two cases:
-
- Strong-field: is significantly large, making it energetically advantageous to fill the orbitals before the orbitals.
- Weak-field: is small, which makes it energetically preferable to fill the orbitals before the orbitals are entirely occupied.
The diagram below shows the ground state configuration of 
-complexes, where 
, for both the strong-field and weak-field cases. For instance, it is energetically favourable for a 
-complex to adopt the 
configuration if is large. When is small, the more stable configuration is 
, where the electrons are in separate orbitals with parallel spins. Complexes with configurations of 3 or more unpaired spins are classified as high-spin complexes, while configurations of less than 3 unpaired spins are known as low-spin complexes.
The above MO diagram is useful for studying charge transfer transitions of an octahedral complex. Charge transfer transitions occur between MOs that are mostly metal in character and those that are mostly ligand in character. These transitions depend on the type of ligand: they occur only when the metal is bound to ligands that are 
-donors or -acceptors. For instance, the transition from 
to is known as a ligand to metal charge transfer (LMCT). If we’re interested in studying 
transitions, which are electronic transitions that occur between MOs that are mostly metal in character, a correlation diagram comes in handy.
