The coordination chemistries of the potential tetradentate ligands N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-ethylenediamine, H-4[L-1], the unsaturated analogue glyoxal-bis(2-hydroxy-3,5-di-tert-butylanil), H-2[L-2], and N,N'-bis-(2-hydroxy-3,5-di-tert-butylphenyl)-2,2-dimethylpropylenediamin e, H-4[L-3], have been investigated with nickel(II), palladium(II), and copper(II). The complexes prepared and characterized are [Ni-II(H3L1)(2)] (1), [Ni-II(HL2)(2)].5/8CH(2)Cl(2) (2), [Ni-II(L-3..)] (3), [Pd-II(L-3..)][Pd-II(H2L3) (4), and [Cu-II(H2O)(L-4)] (5), where (L-4)(2-) is the oxidized diimine form of (L-3)(4-) and (L-3..)(2-) is the bis(o-iminosemiquinonate) diradical form of (L-3)(4-), The structures of compounds 1-5 have been determined by single crystal X-ray crystallography. In complexes 1 and 2, the ligands (H3L1)(-) and (HL2)(-) are tridentate and the nickel ions are in an octahedral ligand environment. The oxidation level of the ligands is that of an aromatic o-aminophenol. 1 and 2 are paramagnetic (mu(eff) similar to 3.2 mu(B) at 300 K), indicating an S = 1 ground state. The diamagnetic, square planar, four-coordinate complexes 3 and [Pd-II(L-3..)] in 4 each contain two antiferromagnetically coupled o-iminobenzosemiquinonate(1-) T radicals. Diamagnetic [Pd-II(H2L3)] in 4 forms an eclipsed dimer via four N-H...O hydrogen bonding contacts which yields a nonbonding Pd...Pd contact of 3.0846-(4) A. Complex 5 contains a five-coordinate Cull ion and two o-aminophenolate(1-) halves in (L-4)(2-). The electrochemistries of complexes 3 and 4a ([Pd-II(L-3..)] of 4) have been investigated, and the EPR spectra of the monocations and -anions are reported.