Further insights on the high-low spin interconversion in nickel(II) tetramine complexes. Solvent and temperature effects

The spin interconversion equil. involving the [NiII(cyclam)]2+ complex was studied in a variety of polar solvents, at varying temps. The greater the donor tendencies of the solvent, the higher the endothermicity of the high-to-low-spin conversion. In particular, a pos. linear relation exists between ΔH° and Gutmann's Donor No. (DN). In the same way, higher donor tendencies of the solvent favor the occurrence of the NiII-to-NiIII oxidn. process and neg. linear relation was found between the E1/2(NiIII/NiII) and DN. General behavior is related to the intensity of the metal-solvent axial bonds in the octahedrally elongated cyclam complexes (of both NiII and NiIII).