TERTIARY-POLY-AMINE LIGANDS AS STABILIZERS OF TRANSITION-METAL COMPLEXES WITH UNCOMMON OXIDATION-STATES

Tertiary-amine ligands are known to be poorer sigma donor ligands than the corresponding primary- or secondary-amine ligands. They are known to shift the redox potentials of given couples to the anodic directio n relative to the corresponding complexes with primary- or secondary-a mine ligands. A review of data in the literature and of recent results on nickel complexes with tetra-aza-macrocyclic ligands and copper com plexes with open chain polyamine ligands suggests that the major sourc e for these effects is the poorer solvation of the complexes with the tertiary-amine complexes due to the lack of hydrogen bonding between t he complexes and the solvent, or the counter ions. Thus the stabilisat ion of low valent transition metal complexes by tertiary-amine ligands is due to thermodynamic reasons. On the other hand, tertiary-amine-ma crocyclic ligands stabilise high valent complexes because the route to the formation of imine groups is kinetically inhibited in these compl exes.