Contrast agents first coordination sphere

In the case of Mn11 complexes as potential MRI contrast agents, the presence of a water molecule in the first coordination sphere is crucial to obtain reasonable relaxivity. Given the lower spin of Mn11, the outer sphere contribution to the overall relaxivity is lower than for Gdm complexes. Thus, for complexes that lack inner sphere water the relaxivity is less than 50% as compared to complexes with coordinated water (233-235). 

All Gd(III) chelates approved for contrast agent application have one inner sphere water molecule. The inner sphere proton relaxivity is linearly proportional to the hydration number q (Eq. (5)), thus a higher q would result in increased relaxivities. However, ligands that leave space for more than one water molecule in the first coordination sphere form complexes of reduced stability,... 

The four examples given above illustrate the effect of water exchange or more exactly proton exchange rate and of the number of water molecules in the first coordination sphere of the paramagnetic metal ion on the water proton relaxation rate. Nonetheless, the relaxivity of a contrast agent can also be strongly... 

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