The octamine cage 5, L, incorporates Ni(II), Cu(II), and Zn(II) in aqueous solution by a fast and reversible process. Equilibrium studies indicated the formation of metal complexes of protonated forms of the ligands, i.e. M(II)(LH(2))](4+) and [M(II)(LH)](+), and of the neutral ligand, [M(II)(L)](2+). The crystal structure of a complex of the monoprotonated ligand, [Ni(II)(LH)](ClO(4))(3), has been determined by single-crystal X-ray crystallography. The complex salt (C(18)H(43)N(8)Cl(3)NiO(12).H(2)O) crystallizes in the orthorombic Pbca space group, with cell constants a = 14.173(2) Å, b = 14.383(1) Å, c = 30.622(3) Å, V = 6242(1) Å(3), and Z = 8. The Ni(II) ion is coordinated to six of the eight available nitrogen atoms, in a very distorted octahedral stereochemistry: of the two uncoordinated donor atoms, a tertiary nitrogen atom and an adjacent secondary one, it is the latter that is protonated. The easy access of protons to uncoordinated amine groups of the cage accounts for the fast demetalation of [Ni(II)(L)](2+) and [Cu(II)(L)](2+) in acidic solution, which was investigated by stopped-flow spectrophotometry. Dependence of k(obs) upon [H(+)] for [Ni(II)(L)](2+) and upon [H(+)](2) for [Cu(II)(L)](2+) indicated that the protonation of uncoordinated nitrogen atoms of the cage is the key step of the demetalation process.