Fig. 4. Coordination sphere engineering approaches developed for square-planar Pd^II systems. (a) Pd^II cations form with four pyridine-based ligands, devoid of steric hindrance, standard homoleptic [Pd₂L₄] cages; (b) the 1 : 1 combination of picolyl ligands with substituents in 3- and 5-positions leads to heteroleptic [Pd₂L^A₂L^B₂] cages; (c) quinoline donors are characterized by steric repulsion between neighbouring H atoms (red colour) and allow to form bowl-like [Pd₂L₃Solv₂] species (two isomeric forms shown); (d) naphthyridine donors increase steric hindrance even further and lead to the formation of [Pd₂L₂Solv₄] rings.