Selectivity of tetra-aza macrocycles towards metal spin states. The high-spin–low-spin equilibrium for nickel( II ) complexes with 13- and 14-membered ligands

Nickel(II) complexes with 13- and 14-membered tetra-aza macrocycles exist in aqueous solution as an equilibrium mixture of a blue high-spin and a yellow low-spin species: the blue-to-yellow conversion is endothermic and predominance of the yellow form stems from a large positive entropy term, due to the release of two or more water molecules. The energy of the in-plane Ni–N interactions for low-spin complexes with 12- to 15-membered macrocycles is related to the size of the aperture in the cyclic ligand.