The reactions of Cd2+ and Zn2+ with the pyridine-arm isoindoline ligand 4'-MeLH = 1,3-bis[2-(4-methylpyridyl)-imino] isoindoline produced the series of octahedrally coordinated complexes M(4'-MeL)(2), [M(4'-MeLH)(2)](2+), and [M(4-MeL)(4'-MeLH)](+). The complexes M(4'-MeL)2 resulted from reactions of the respective metal perchlorates with deprotonated ligand, whereas the complexes [M(4'-MeLH)(2)](ClO4)(2) resulted from reactions with ligand in the absence of added base. The mixed-ligand complexes [M(4'-MeL)(4'-MeLH)](+) were generated in solution by reactions of equimolar quantities of M(4'-MeL)(2) and [M(4'-MeLH)(2)](2+). Whereas [Cd(4'-MeL)(4'-MeLH)](+) is stable in solution, [Zn(4'-MeL)(4'-MeLH)](+) converts to and establishes equilibrium with the tetrahedrally coordinated, trinuclear complex [Zn-3(4'-MeL)(4)](2+). The complexes Cd(4'-MeL)(2) (1), Zn(4'-MeL)(2) (2), and [Cd(4'-MeL)(4'-MeLH)]CIO4 (5) were characterized by single-crystal X-ray diffraction, with the latter complex being shown to contain 4'-MeLH coordinated as a protonated iminium zwitterionic ligand. The [M(4'-MeLH)(2)](2+) and [M(4'-MeL)(4'-MeLH)](+) complexes are tautomeric in solution because of the shuttling of the iminium protons between imine N atoms. The rate of prototropic tautomerism in [Cd(4'-MeLH)(2)](+) was followed by H-1 NMR spectroscopy. Over the temperature range 276-312 K, a linear Eyring plot with the activation parameters Delta G(+/-) = 16.0 +/- 0.1 kcal/mol, Delta H-+/- 2.9 +/-0.1 kcal/mol, and Delta S-+/- = - 44.0 +/- 0.3 cal/mol.K was obtained.