The preparation and characterization of a pi-cation radical derivative of a vanadyl porphyrinate is described. [VO(OH2)(OEP(.))]SbCl6 is prepared by chemical oxidation of [VO(OEP)]; the formulation has been confirmed by a single-crystal X-ray structure determination. The coordinated water molecule of the unusual six-coordinate vanadyl complex is derived from solvent. Axial bond distances are the following : V-O = 1.578(4) Angstrom and V-O(H2O) = 2.473(8) Angstrom; the vanadyl ion is displaced 0.46 Angstrom from the 24-atom mean plane. The average V-N-p distance is 2.063 Angstrom. Crystal data : a = 15.530(2) Angstrom, b = 14.586(4) Angstrom, c = 18.965(3) Angstrom, and beta = 106.20(1)degrees, monoclinic, space group P2(1)/n, V = 4125.4 Angstrom(3), Z = 4, [VO2N4C36H45]SbCl6, R(1) = 0.053 and R(2) = 0.064 for 4707 observed data. EPR spectra have been measured for [VO(OH2)(OEP(.))]SbCl6 in the solid (powder) state, in a single crystal, and in fluid and frozen solution. The solution-state EPR spectra of [VO(OH2)(OEP(.))]SbCl6 are quite distinct, with vanadium hyperfine lines and with differing spectra in fluid and frozen solution. These solution spectra are identical to those reported earlier by others. In the solid state, the EPR spectrum of a polycrystalline sample consists of a single broad line with g = 1.99; single-crystal spectra also show vanadium hyperfine-splitting consistent with V-V coupling. The solid-state EPR intensity increases with increasing temperature. [VO(OH2)(OEP(.))]SbCl6 has also been characterized by a detailed temperature-dependent (6-300 K) magnetic susceptibility study. Satisfactory fits of the temperature-dependent moments are obtained from a model in which the vanadyl electron is ferromagnetically coupled to the porphyrin cation electron (J(v-r) = 63 cm(-1)) and two radical spins are antiferromagnetically coupled (J(r-r) = -139 cm(-1)). This model also gives a satisfactory prediction of all solid-state EPR properties. The intramolecular ferromagnetic coupling is consistent with the principle of cospatial, orthogonal magnetic orbitals. The intermolecular antiferromagnetic coupling appears to arise from a novel, steplike orientation of pairs of (OEP(.)) radicals.