Before revision: They were introduced for study omplexes of ions like Ti3+, that have a single unpaired electron in the metal ion's valence electron shell and to complexes of ions like Cu2+ which have a single "vacancy" in the valence shell.
After revision: They were introduced for studying complexes of ions like Ti3+, that have a single unpaired electron in the metal ion's valence electron shell , and, complexes of ions like Cu2+ which have a single "vacancy" in the valence shell.