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

NiL2+ (L = 1,4,7,10-tetrazacyclotridecane) was prepd. and thermodn. parameters for its soln. equil. were detd. and compared with those for [NiL1]2+ (L1 = 1,4,8,11-tetrazatetradecane). These complexes exist in aq. soln. as an equil. mixt. 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 pos. entropy term, due to the release of 2 or more H2O mols. 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.