Quadridentate ligand cyclam

As far as quadridentate ligands are concerned, the easiest attainment of the Ni state, denoted by the less positive redox potential, is observed with the 14-membered macrocycle, 5, known as cyclam. Both contraction and expansion of tetramine ring atomicity, to 12 and to 16, respectively, makes the potential more positive, thus destabilizing Ni with respect to Ni Moreover, the isomeric 14-membered macrocycle, 6 (isocyclam) stabilizes the Ni state to a much lesser extent than cyclam. Notice that the two macrocyclic complexes simply differ in the sequence of the five-... 

Quadridentate Ligands. The effect of extent of chelation on aquation rates has been studied for the compounds raK5-[Co(N02)2(LLLL)]+, where LLLL = (NH3)4, (en>2, (pnja, or cyclam (5), and ci5-[Co(N02)2(LLLL)]+, where LLLL = (NHg)4, (en)a, or trien (6). Rates of aquation increase as the number and size of chelate rings decrease, which is consistent with a dissociative mechanism. Rates of aquation are also sensitive to steric crowding around the leaving group. This is well illustrated by the 10 -fold difference in aquation rates of [Co(C03)(LLLL)]+ when LLLL = (en)a or tet-b (7), and by rate variations in the series... 

Cyclam, tet a, tet b, and trans[14]diene are a group of macrocyclic quadridentate nitrogen donor ligands. 

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