The molecular structure of the Ni(II)OEP+ClO4- pi cation radical is reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin). The vibrational spectrum of a single crystal, recorded with an FT-IR micrcospectrometer, shows the (2)A(lu), HOMO occupancy marker band predicted for an OEP cation radical. The radical crystallizes as a cofacial pi-pi dimer, (NiOEP+ClO4-)(2) . 24=H2Cl2, in an eclipsed configuration with the following parameters : mean interplanar separation 3.36 Angstrom, Ni-Ni = 3.41 Angstrom, Ct-Ct = 3.46 Angstrom, lateral shift = 0.83 Angstrom, translational slip angle = 13.9 degrees, and zero rotation of equivalent N-Ni-N axes. This dimeric configuration differs significantly from that of a related (NiOEP+ClO4-)(2) . 8CH(2)Cl(2) dimer and from those of several other metallo (M) OEP radical dimers described previously. The present results offer exceptions to generalizations recently proposed regarding the geometry and extent of pi-pi interactions in fully and partially oxidized MOEP+ dimeric pi cation radicals such as (MOEP+)(2) and (MOEP)(2)(+). Indeed, the configuration and geometry of the (NiOEP+ClO4-)(2) dimer reported here are more akin to those of a half-oxidized dimer (NiOEP)(2)+SbCl6- than to those of a fully oxidized (NiOEP+ClO4-)2 dimer reported previously (Scheidt et al. Inorg. Chem. 1996, 35, 7500). The present results do demonstrate that identical porphyrin pi cation radicals can aggregate in and adopt more than one unique cofacial geometry with different conformations, pi-pi overlaps, and intermolecular spacings. Such multiple configurational architectures may help explain why the oxidized bacteriochlophyll dimers (special pairs) of bacterial photosynthetic reaction centers exhibit varying unpaired spin density profiles.