Abstract:We report two novel macrocyclic ligands containing a cyclen unit, a methyl group, a picolinate arm and two acetate pendant arms attached to two nitrogen atoms of the macrocycle either in trans (1,7-H3Medo2ampa) or in cis (1,4-H3Medo2ampa) positions. These ligands provide eight-coordination to the Ln 3+ ions leaving a coordination position available for a water molecule that occupies a capping position in the twisted square antiprismatic polyhedron (1,4-H3Medo2ampa) or one of the positions of the square antiprism (1,7-H3Medo2ampa). The charge neutral [Gd(1,7-Medo2ampa)] complex presents an unprecedentedly low water exchange rate (kex 298 = 8.8 10 3 s -1 ), while water exchange in [Gd(1,] is three orders of magnitude faster (kex 298 = 6.6 10 6 s -1 ). These results showcase the labile capping bond phenomenon: a ligand occupying a capping position is hindered by the environment and thus is intrinsically labile.