Does a macrocyclic effect exist in the blue-to-yellow conversion of nickel(II) complexes with tetra-aza ligands?

A thermodn. study was made of the blue-to-yellow conversion, [NiL(H2O)2]2+ .dblharw. [NiL]2+ + 2H2O, of the Ni complexes, where L = 1,4,8,11-tetraazacyclotetradecane (cyclam) and its linear analogs, 1,4,8,11-tetraazaundecane (2,3,2-tet) and 1,5,8,12-tetraazadodecane (3,2,3-tet), in order to ascertain whether or not the cyclic nature of the ligand might affect the position of equil. At 25° in 0.1M NaClO4 [Ni(cyclam)]2+ exists as 71% yellow square-planar form and 29% blue octahedral form (K = 2.47). From K the ΔH° and ΔS° were calcd. Under the same conditions [Ni(2,3,2-tet)]2+ exists as 22% yellow form; [Ni(3,2,3-tet)]2+ as 8% yellow. The enhanced stability of the yellow [Ni(cyclam)]2+ depends on a very favorable entropy term. As the size of the aliph. part of the tetramine ligand increases, the larger steric repulsions between methylene groups and coordinated H2O will favor the mobilization-dehydration process and ΔS° should increase in the series 2,3,2-tet < 3,2,3-tet < cyclam, and this agrees with the exptl. results. The ability of cyclam to stabilize the square planar yellow form does not result from any special cyclic effect but depends only on steric interactions. The mech. consequences of cyclization actually hinder the macrocyclic ligand in its coordination to low-spin Ni(II).