The synthesis of a new monoheteroporphyrin, 5,20-diphenyl-10,15-bis(p-tolyl)-21-selenaporphyrin (SeDPDTPH), is reported. The 21-selenaporphyrin has been characterized by H-1 NMR, C-13 NMR, mass spectrometry, and W-visible spectra and an X-ray structural analysis. The free base selenaporphyrin SeDPDTPH crystallizes in the monoclinic space group P2(1)/(n) with a = 19.848(3) Angstrom, b = 8.8410(14) Angstrom, c = 20.503(4) Angstrom, beta = 103.375(12)degrees at 125 K with Z = 4. Refinement of all 4577 unique reflections and 453 parameters yielded R(1) = 0.096 (based on F-2). The presence of selenium atom elongates the macrocycle along the N(1)-N(3) axis when compared to a regular porphyrin. The n delocalization pattern is altered in the selenophene moiety in relation to the free selenophene. SeDPDTPH undergoes a two-step proton addition in solution. Mono- and dication formation results in distortion of the planar 21-selenaporphyrin structure. The monocation structure is solvent dependent as shown by the H-1 NMR titration experiments. Insertion of Ni(II) into 21-selenaporphyrin yields Ni-II(SeDPDTP)Cl (S = 1, mu(eff) = 3.3 mu(B)) The electronic spectrum of this complex is porphyrin-like with a strong Soret band at 433 nm. The H-1 NMR spectra of the high-spin nickel(II) complexes of 21-selenaporphyrin have been recorded and assigned by means of the selective deuteration, line width considerations, and a 2D COSY experiment. The characteristic pattern of pyrrole (downfield) and selenophene (upfield) resonances has been established. Direct sigma-pi spin density transfer has been proposed to explain the upfield shift of the selenophene protons. Imidazole replaces the chloride ligand to form five- and six-coordinate complexes. The spectroscopic and chemical properties of Ni-II(SeDPDTP)Cl resemble those of nickel(II) complexes with 21-thiaporphyrin. To account for these similarities, we suggest that the selenophene moiety is bent out of the porphyrin plane in Ni-II(SeDPDTP)Cl. Such a geometry allows metal ion to interact with selenium of selenophene in a side-on fashion.