Metal complex formation of the two cyclic triamines 6-methyl-1,4-diazepan-6-amine (Me La) and all-cis-2,4,6-trimethylcyclohexane-1,3,5-triamine (Me(3)tach) was studied. The structure of the free ligands (HxMeLa)(x-1-) and H(x)Me(3)tach(x+) (0 <= X <= 3) was investigated by pH-dependent NMR spectroscopy and X-ray diffraction experiments. The crystal structure of (H(2)Me(3)tach)(P-O3S C6H4-CH3)(2) showed a chair conformation with axial nitrogen atoms for the doubly protonated species. In contrast to a previous report, Me3tach was found to be a stronger base than the parent cis-cyclohexane-1,3,5-triamine (tach); pK(a)-values of H(3)Me(3)tach(3+) (25 degrees C, 0.1 M KCl): 5.2, 7.4, 11.2. The crystal structures of (H3MeLa)(BiCl6) center dot 2H(2)O and (H3MeLa)(ClO4)Cl-2 exhibited two distinct twisted chair conformations of the seven membered diazepane ring. [Co(MeLa)(2)](3+) (cis: 1(3+), trans: 2(3+)), trans-[Fe(MeLa)(2)](3+) (3(3+)), [(MeLa)ClCd(mu(2)-Cl)](2) (4), trans-[Cu(MeLa)(2)](2+) (5(2+)), and [Cu(HMeLa)Br-3] (6) were characterized by single crystal X-ray analysis of 1(ClO4)(3)center dot H2O, 2Br(3) center dot H2O, 3(ClO4)(3) center dot 0.8MeCN-0.2MeOH, 4, 5Br(2)- 0.5MeOH, and 6 center dot H2O. Formation constants and redox potentials of MeLa complexes were determined by potentiometric, spectrophotometric, and cyclovoltammetric measurements. The stability of [M-II(MeLa)(2+)-complexes is low. In comparison to the parent 1,4-diazepan-6-amine (La), it is only slightly enhanced. In analogy to La, MeLa exhibited a pronounced tendency for forming protonated species such as [Mn-II(HMeLa)](3+) or [M-II(MeLa)(HMeLa)](3+) (see 6 as an example). In contrast to MeLa, Me3tach forms [(ML)-L-II](2+) complexes (M = Cu, Zn) of very high stability, and the coordination behavior corresponds mainly to an "all-or-nothing" process. Molecular mechanics calculations showed that the low stability of La and MeLa complexes is mainly due to a large amount of torsional strain within the pure chair conformation of the diazepane ring, required for tridentate coordination. This behavior is quite contrary to Me(3)tach and tacn (tacn = 1,4,7-triazacyclononane), where the main portion of strain is already preformed in the free ligand, and the amount, generated upon complex formation, is comparably low.