Spectrochemical series (ligand field strength) (Weak-field ligands left; strong-field right: increases Δo; exact order exam-dependent but)
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Weak-field ligands left; strong-field right — increases Δo; exact order exam-dependent but trends must be known
When to use this
π-acid ligands like CO stabilize eg* via backbonding — very large Δo; halides weak σ donors.