The spectrochemical series ranks ligands by their ability to split the d-orbitals of a transition metal centre — quantified by the crystal field splitting parameter Δ. Weak-field ligands such as I⁻ and Br⁻ produce small Δ values, favouring high-spin electron configurations, while strong-field ligands such as CO and CN⁻ produce large Δ values that force electrons to pair in lower-energy orbitals, giving low-spin complexes. The strength of a ligand is determined by its donor mechanism: σ-only donors (e.g. NH₃) sit in the middle of the series, π-donor ligands (halides) weaken the field, and π-acceptor ligands (CO, CN⁻, bipy) strengthen it through back-bonding with metal d-orbitals. Clicking any ligand in the series displays its IUPAC name, charge, denticity, donor type, spin tendency, and representative example complexes. The filter bar allows the series to be narrowed by ligand category.