The chemical species (an ion or a compound) which will bond to a metal ion using lone pairs of electrons.
The term "complexing agent" was found in the following pages:
Iron Species Determination based on HPLC with a Short Column and Detection by ICP-OES | EVISA's News
Thiomersal in influenza vaccine: more than just an adjuvant | EVISA's News
Uptake of mercury species by fruit flies | EVISA's News