1. I- < Br- < SCN- < Cl < S2- < F- < OH- ≈ urea < OH2- < H2O < NCS- < EDTA4- < NH3 < en < NO-2 < CN < CO
The above series is known as spectrochemical series.
2. The ligands present on the right side of the series such as carbonyl causes relatively larger crystal field splitting and are called strong ligands (or) strong field ligands.
3. The ligands on the left side are called weak field ligands and causes relatively smaller crystal field splitting.