Octahedral structure of Tc-99m chelates

In the presence of a chelating agent (a compound with electronegative character that is capable of binding radiometal cations), the metal ion is tightly bound in the center of an octahedral structure. It is said to be hexa-coordinated since there are typically 6 binding sites available consisting of O- N-, or P-atoms. The bound complex is called a chelate, e.g., Tc-MDP, In-DTPA. The diagram below represents an octahedral structure for a hexa-coordinated complex.