A ligand with two DO3A units connected by a methylenebis(phosphinate) spacer (ligand L2) and a DOTA analogue with one methylenebis(phosphinate) pendant arm (ligand L1) were prepared, and their mono/dinuclear lanthanide(III) complexes were studied. Their twisted-square antiprismatic (TSA)/square-prismatic (SA) isomers exhibit only one (vertical) configuration of the chiral phosphorus atom, leading to two/six major diastereoisomers for the mono/dinuclear complexes, respectively. All isomers mutually exchange, but in the L2 complexes, the TSA/SA exchange occurs only through macrocycle inversion. Phosphinate rotation exchanging the phosphorus atom chirality was not observed. In the Ln₂-L2 complexes, each unit behaves independently and has one coordinated water molecule up to the Er(III) complex (as determined by Eu(III) fluorescence and ⁸⁹Y NMR). The Ln−Ln distance is about 5.7 Å. Per Gd(III) ¹H relaxivities of the mono/dinuclear complexes are almost identical, revealing no significant Gd−Gd interaction in the dinuclear complex. The molecular relaxivity of the Gd₂-L2 complex (14.4 mM⁻¹ s⁻¹, 0.94 T, 25 °C) is among the highest values measured for a monohydrated dinuclear complex and is almost the same at 0.94 and 7 T. Since the methylene group of the bridge can be synthetically modified, this paves the way to targeted magnetic resonance imaging (MRI) contrast agents (CAs) with high relaxivity at high magnetic fields.