Figure 1.

Molecular stick models of Gd(H₂O)₈³⁺ and Gd(H₂O)DTPA^2-. Note the one inner-sphere water molecule in Gd(H₂O)DTPA^2- that is important for MRI contrast. Mixing of DTPA with Gd³⁺ results in fast formation of a ML complex because of the DTPA ligand is flexible and can “wrap around” the Gd³⁺ ion very quickly. Water exchange is fast so removal of water molecules from the inner coordination sphere of the Gd³⁺ does not hinder the rate at which the complex forms.