In water, the [Pd(H 2 O) 4 ] 2+ complex has a Pd-O equatorial bond distance of 2.00-2.05 Å, with a second solvation shell of 10 waters located between 4.02-4.40 Å. The Pd-O axial distance has been investigated by neutron diffraction and extended X-ray absorption fine structure experiments and with different computational methods. The axial interaction distance is reported to range from 2.5 to 3.0 Å. Ligand exchange in Pd 2+ square planar complexes is suggested to occur via an associative mechanism involving a pentacoordinated trigonal bipyramidal transition state (TS), as suggested by Ligand Field Theory and supported by DFT calculations. While Mg 2+ has a prominent role in biology, Pd 2+ is a key building block in supramolecular chemistry, giving rise to a wide range of metallocages of various sizes and shapes. The interplay between the metal, organic ligands, and solvents determines the final assembled structure. Notably, the labile nature of Pd 2+ -ligand axial interactions is key for self-correction and optimal self-assembly. Pd 2+ -based metal-organic cages are commonly formed in MeCN solvent, although water and DMSO are also widely-used.