Weak Field Ligand
A ligand that produces only a small crystal field splitting, so electrons occupy upper orbitals rather than pairing.
Formula / equation
Δ < P
Unit
cm⁻¹
In depth
When the splitting is less than the pairing energy, Hund's rule prevails and the complex is high spin with the maximum number of unpaired electrons. Weak field ligands are usually pi donors such as the halides, which destabilise the t₂g set.
Examples in the real world
I⁻, Br⁻, Cl⁻ and F⁻; [CoF₆]³⁻ is paramagnetic while [Co(NH₃)₆]³⁺ is not.