The conformational properties of lanthanide(III) complexes with the mono- and biphosphonate analogues of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) are investigated by means of density functional theory (DFT) calculations and NMR spectroscopy Geometry optimizations performed at the B3LYP/6-31G(d) level and using a 46 + 4f(n) effective core potential for lanthanides provide two energy minima corresponding to the square-antiprismatic (SAP) and twisted square-antiprismatic (TSAP) geometries Our calculations give relative free energies between the SAP and TSAP isomers in fairly good agreement with the experimental values The SAP isomer presents the highest binding energy of the ligand to the metal ion, which further increases with respect to that of the TSAP isomer across the lanthanide senes as the charge density of the metal ion increases The stabilization of the TSAR isomer upon substitution of the acetate arms of DOTA by methylenephosphonate ones is attributed to the higher steric demand of the phosphonate groups and the higher strain of the ligand in the SAP isomer A H-1 NMR band-shape analysis performed on the [Ln(DO(2)A(2)P)](3-) (Ln = La and Lu) complexes provided the activation parameters for enantiomerization of the TSAP form of the complexes. The TSAP isomerization process was also investigated by using DFT calculations on the [Lu(DOTA)](-) and [Ln(DO(2)A(2)P)](3-) (Ln = La and Lu) systems. Our results confirm that enantiomerization requires both rotation of the pendant arms and inversion of the four five-membered chelate rings formed upon coordination of the macrocyclic unit According to our calculations, the arm rotation pathway in [Lu(DOTA)](-) is a one-step process involving the simultaneous rotation of the four acetate arms, while in the DO(2)A(2)P analogue, the arm-rotation process is a multistep path involving the stepwise rotation of each of the four pendant arms The calculated activation free energies are in reasonably good agreement with the experimental data A comparison of the experimental C-13 NMR shifts of [Ln(DO(2)A(2)P)](3-) (Ln=La and Lu) complexes and those calculated by using the GIAO method confirms that the major isomer observed in solution for these complexes corresponds to the TSAP isomer.