The structure of (porphinato)nickel(II) [Ni(P)] has been determined by X-ray diffraction and inferred from a combination of single-crystal and solution resonance Raman measurements. The crystal structure reveals a planar porphyrin macrocycle with a pi-pi dimer packing configuration exhibiting a small lateral shift. This group S crystallographic packing arrangement has been suggested to give strong pi-pi interactions between the porphyrin rings on the basis of the small interplanar spacing (3.355 Angstrom) and lateral shift (1.528 Angstrom) between the porphine planes. Strong pi-pi interactions are usually associated with geometrically inequivalent structural parameters such as different metal-nitrogen bond distances, but this is not observed for Ni(P). The average nickel-nitrogen bond distance is 1.951 Angstrom, consistent with planar nickel porphyrins. The root-mean-square out-of-plane displacement from the mean plane of the macrocyclic atoms is 0.019 Angstrom, consonant with the observed very slight ruffling of the macrocycle. A salient feature of the resonance Raman spectra of Ni(P) in solution is the apparent sidebands of some structure-sensitive lines. This observation was interpreted previously as resulting from an equilibrium between planar and nonplanar conformers in solution. However, the similarities of the resonance Raman spectra of Ni(P) in the single crystal and in solution suggest that Ni(P) exists only in the planar conformation. This conclusion is corroborated by solution resonance Raman spectra of the four-coordinate (porphinato)copper(II) and (porphinato)cobalt(II), which are more likely than Ni(P) to be planar because of their larger central metals, yet they also show the sidebands of the same structure-sensitive lines. Crystal data : [C20H12N4]Ni; a = 10.1066(7) Angstrom, b = 11.945(9) Angstrom, c = 12.229(2) Angstrom, beta = 101.56(3)degrees, Z = 4, V = 1446.4(11) Angstrom(3), space group P2(1)/c; 3084 unique observed data; refinement converged to final values of R(1) = 0.039, wR(2) = 0.090; all measurements at 127(2) K.