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

Article abstract of DOI:10.1021/ic50204a052

The enthalpies of formation of nickel complexes with tetraaza macrocyclic ligands of varying ring size, from 12 to 15 members, have been determined by destroying the complexes in aqueous solution with alkaline cyanide. The octahedral-square-planar equilibria have been 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 maximum 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 two square-planar complexes is rationalized in terms of expected ligand transformations.