Crystal Field Theory (CFT)
In octahedral coordination complexes, electrostatic repulsion splits the five degenerate $d$-orbitals into two groups:
- Lower energy triply degenerate: $t_{2g}$ ($d_{xy}, d_{yz}, d_{zx}$)
- Higher energy doubly degenerate: $e_g$ ($d_{x^2-y^2}, d_{z^2}$)
- Crystal field splitting energy: $\Delta_o$.
- Strong field ligands (e.g., $CN^-, CO$) cause large $\Delta_o > P$ (pairing energy), forcing low-spin complexes.
