Gadolinium forms a large number of highly ionic chelate complexes. Current commercial Gd-chelates for MRI applications are nine coordinate in which the ligand (DTPA, DOTA, HP-DO3A) occupies eight binding sites and the ninth coordination site at the metal ion is occupied by a water molecule.
Gadolinium is a member of the lanthanides (f-elements). Because of its symmetric 4f7 (seven upaired electrons) electronic configuration and its long electronic relaxation rate it is the most commonly used paramagnetic ion for MRI contrast agents. Due to its similarity in size and in chemical properties to Ca(II), gadolinium is a toxic heavy metal. Gadolinium forms a large number of highly ionic chelate complexes. Because of its large size, Gd(III) usually forms complexes with high coordination numbers in aqueous media. Current commercial Gd-chelates for MRI applications are nine coordinate in which the ligand occupies eight binding sites and the ninth coordination site at the metal ion is occupied by a water molecule. The coordination geometry for nine coordinate complexes are tricapped trigonal prism or, in case of chelate complexes, capped square antiprism. According to the Solomon-Bloembergen-Morgan theory of relaxivity, in order to achieve maximum water proton relaxivity it is necessary to maximize the number of coordinated water molecules, and to optimize the water exchange rate and rotational correlation time. In addition, as dissociation of the complex under biological conditions is undesirable, the chelator must have sufficient thermodynamic and kinetic stability. Polyamimo-polycarboxylate ligands such as DTPA, DOTA, HP-DO3A can safely chelate gadolinium(III) and are used as as chelators for gadoliniumin the clinical practice.