\underset{\phantom{\rule{3em}{0ex}}\text{a few ligands of the spectrochemical series, in order of increasing field strength of the ligand}\phantom{\rule{3em}{0ex}}}{\overset{\phantom{\rule{3em}{0ex}}{\text{I}}^{-}<{\text{Br}}^{-}<{\text{Cl}}^{-}<{\text{F}}^{-}<{\text{H}}_{2}\text{O}<{\text{C}}_{2}{\text{O}}_{4}{}^{2-}<{\text{NH}}_{3}<en<{\text{NO}}_{2}{}^{-}<{\text{CN}}^{-}\phantom{\rule{3em}{0ex}}}{\to }}