Formation constants of Mn2+, Co2+, Ni2+, CU2+, and Zn2+ complexes formed in aqueous solutions with 12-membered and 13-membered macrocycles have been determined, and the solution electronic spectra have been studied. The 12-membered macrocycle, abbreviated as (12edtaen)H-2, is 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclododecanediacetic acid and the 13-membered macrocycle, abbreviated as (13edtapn)H-2, is 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotridecanediacetic acid. The structures of [Zn(C12H18N4O6)]. 4H(2)O and [Mn(C13H20N4O6)(H2O)](2) . 7H(2)O have been determined by single-crystal X-ray analyses. The zinc complex crystallized in the monoclinic space group P2(1)/n with a = 10.205(2) Angstrom, b = 9.599(2) Angstrom, c = 19.431(3) Angstrom, beta = 100.268 (2)degrees, and Z = 4. The coordination geometry around the zinc atom is distorted octahedral with five donor atoms (two amine nitrogen, an amide oxygen, and two acetate oxygen atoms) from one ligand molecule and an acetate oxygen atom from a neighboring zinc chelate. The zinc atoms are linked by Zn-O-C-O-Zn bonds and form a one-dimensional array. The manganese complex crystallized in the orthorhombic space group Peen with a = 16.325(1) Angstrom, b = 17.468(1) Angstrom, c = 15.144(1) Angstrom, and Z = 4. The coordination geometry around the manganese atom is a highly distorted trigonal prism with five donor atoms from a ligand molecule and an oxygen atom from a water molecule. The formation constants of the ML species of Ni2+ and CU2+ With (12edtaen)(2-) are significantly higher than those of the corresponding complexes with (13edtapn)(2-). The MLH(-2) species of Co2+, Ni2+, and CU2+ formed with (13edtapn)(2-) in the basic region have greater formation constants than the corresponding MLH(-1) species. The solution electronic spectra of the MLH(-2) species of these complexes are quite different from those of the corresponding ML species, indicating that conversion of coordination geometry occurs as a result of deprotonation of amide nitrogen atoms. This conversion has been confirmed by an electron spin echo envelope modulation experiment for the CU2+ complex. No significant spectral changes are observed with the M(12edtaen) complexes. In these complexes the mixed-ligand complexes, M(12edtaen)(OH)(n) (n = 1 or 2), are formed in the basic region, and the complexes with n = 2 are much less stable than the corresponding complexes with n = 1. These large differences in the properties of the metal complexes of the two macrocyclic ligands are caused by a difference of only one -CH2- group in the ligand ring system.