The crystal structures of meso-NH2-OEPH2, Ni-II(meso-NH2-OEP), and Cu-II(meso-NH2-OEP) (where OEP is the dianion of meso-amino-octaethylporphyrin) have been determined to examine the effects of the meso-substituent on the geometry of the ligand. Cull (meso-NH2-OEP) has a nearly planar geometry while the free ligand itself and Ni-II(meso-NH2-OEP) have ruf conformations. Ni-II(meso-NH2-OEP) is much less reactive toward oxidation than are (py)(2)Fe-II(meso-NH2-OEP), CIFeIII(meso-NH2-OEP), or Ni-II(meso-HO-OEP), which all undergo oxidation in pyridine solution when exposed to dioxygen. Treatment of Ni-II(meso-NH2-OEP) with iron(III) chloride in chloroform solution does result in oxidation of the ligand in two separate processes. One involves oxygenation at the trans-meso position, while the other results in ring cleavage and removal of the amino function. The open-chain tetrapyrrole complex, Ni-II(OEB-CO2Et), has been characterized by single-crystal X-ray diffraction and shown to contain a helical ligand with a four-coordinate nickel ion.