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Crystal Field Theory (CFT) explains bonding in coordination compounds using a purely electrostatic model. Ligands are treated as point charges (or point dipoles) that create an electric field around the central metal ion.

In a free metal ion, all five d-orbitals are degenerate (equal energy). When ligands approach, this degeneracy is broken. The pattern of splitting depends on geometry:

Octahedral: Ligands along x, y, z axes push dx2-y2 and dz2 (eg set) to higher energy (they point directly at ligands). dxy, dyz, dxz (t2g set) go lower. Splitting = $Delta_{oct}$. Barycentre maintained: t2g stabilised by 0.4 $Delta_{oct}$, eg destabilised by 0.6 $Delta_{oct}$.

Tetrahedral: Splitting is inverted. e set (dx2-y2, dz2) goes lower, t2 set (dxy, dyz, dxz) goes higher. $Delta_{tet}$ = $\frac{4}{9}$ $Delta_{oct}$. Small splitting means virtually always high spin.

Square planar: Energy order — dxz, dyz < dz2 < dxy < dx2-y2 (highest). Large splitting of dx2-y2 favours this geometry for d8 with strong field ligands.

CFSE (Crystal Field Stabilisation Energy) = sum of (-0.4 x $n_{t2g}$ + 0.6 x $n_{eg}$) x $Delta_{oct}$. Maximum CFSE: d3 in high spin (-1.2 $Delta_{oct}$) and d6 in low spin (-2.4 $Delta_{oct}$). Zero CFSE: d0, d5 high spin, d10. CFSE determines thermodynamic stability, kinetic lability, colour preferences, and site preferences in crystal structures.