A microcalorimetric determination of the enthalpies of formation in solution of nickel(II) complexes with tetraaza macrocyclic ligands of varying size

The heats of formation of Ni complexes with tetraaza macrocyclic ligands of varying ring size, (12-15 members) were detd. by destroying the complexes in aq. soln. with alk. cyanide. The octahedral-square-planar equil. were investigated, and the results are presented as ΔH° for both octahedral, blue and when possible, square-planar, yellow species. Like the Cu(II) system, the blue, octahedral series exhibits a max. in ΔH° with [14]aneN4. This is discussed in terms of the relative size of the ligand and the metal ion. The unexpected reversal of ΔH° for the 2 square-planar complexes is rationalized in terms of expected ligand transformations.