A series of the most common chelators used in magnetic resonance imaging (MRI) and in radiopharmaceuticals for medical diagnosis and tumour therapy, H₄dota, H₄teta, H₈dotp and H₈tetp, is examined from a chemical point of view. Differences between 12- and 14-membered tetraazamacrocyclic derivatives with methylcarboxylate and methylphosphonate pendant arms and their chelates with divalent first-series transition metal and trivalent lanthanide ions are discussed on the basis of their thermodynamic stability constants, X-ray structures and theoretical studies.

Cyclen-based ligands containing two picolinate pendant arms form Gd ³⁺ complexes remarkably stable and inert with respect to metal ion dissociation.