spectrochemical series

Not all ligands push the d orbitals apart equally. Some open a wide splitting gap, others only a narrow one. If you rank ligands by how big a delta they produce on the same metal, you get a fixed pecking order that holds remarkably well from one complex to the next. That ranking is the spectrochemical series, and chemists got it by reading delta straight off absorption spectra — hence the name.

A common run of the series, from weakest field (small delta) to strongest (large delta), is: iodide < bromide < chloride < fluoride < hydroxide < water < ammonia < ethylenediamine < bipyridine and phenanthroline < cyanide and carbon monoxide. So CO and CN- are strong-field ligands that split d orbitals widely, while halides are weak-field and split them only a little. A blunt electrostatic model cannot explain this order at all — it would expect the most negative ions to split hardest, yet neutral CO outsplits charged fluoride. The real explanation involves pi bonding: pi-acceptor ligands like CO and CN- pull electron density out of the metal t2g orbitals through their empty pi orbitals, which deepens t2g and so enlarges delta, while pi-donor ligands like the halides do the reverse and shrink delta.

The series is the practical key to predicting transition-metal behavior. Knowing where a ligand sits tells you roughly how big delta will be, and that, weighed against the pairing energy, predicts whether a complex is high-spin or low-spin, what color it will be, and how stable. It is also why the same metal ion changes color when you swap its ligands — adding ammonia to a pale blue copper solution deepens it to royal blue, because ammonia sits higher in the series than water and widens delta.