Coordination Compounds · Crystal Field Theory

d-Orbital Splitting

Ligands approaching a metal ion break the five equal d-orbitals into two energy levels, split by Δ. A strong-field ligand splits them so far that electrons would rather pair up than climb — giving a low-spin ion; a weak field lets them spread out high-spin. That choice sets the number of unpaired electrons, the magnetism and even the colour.

↑ electron spin ┃ Δ splitting octahedral: t₂g below, eₒ above

Complex

d-electrons d6 6
Low spin
absorbs ~490 nm
0
unpaired e⁻
diamagnetic
magnetism
−2.4
CFSE (× Δ)
strong
field strength
Δ > P → pair up (low spin)

Try this

Same metal, two ligands. Set d⁶ (like Fe²⁺): with H₂O it's high-spin and paramagnetic; switch to CN⁻ and the big Δ forces low-spin — all paired, diamagnetic. That is [Fe(CN)₆]⁴⁻ vs [Fe(H₂O)₆]²⁺. Then switch to tetrahedral: Δt is only 4/9 of Δo, so it's always high-spin.