The search for more biocompatible alternatives to Gd 3+-based MRI agents,a nd the interest in 52 Mn for PET imaging call for ligands that form inert Mn 2+ chelates.G iven the labile nature of Mn 2+ ,h igh inertness is challenging to achieve.The strongly preorganized structure of the 2,4-pyridyldisubstituted bispidol ligand L 1 endows its Mn 2+ complex with exceptional kinetic inertness.I ndeed, MnL 1 did not showa ny dissociation for 140 days in the presence of 50 equiv.o fZ n 2+ (37 8 8C, pH 6), while recently reported potential MRI agents MnPyC3A and MnPC2A-EA have dissociation half-lives of 0.285 ha nd 54.4 hu nder similar conditions.I na ddition, the relaxivity of MnL 1 (4.28 mm À1 s À1 at 25 8 8C, 20 MHz) is remarkable for am onohydrated, small Mn 2+ chelate.I nvivo MRI experiments in mice and determination of the tissue Mn content evidence rapid renal clearance of MnL 1 .Additionally, L 1 could be radiolabeled with 52 Mn and the complex revealed good stability in biological media.