The low-valent titanium induced reductive coupling of metal(II) 5-formyl-octaethylporphyrin (OEP) (2,3) leads to the formation of ethene-bridged bis-porphyrins. Structural studies on the main products of the coupling of Cu(II)- (2) and Ni(II)5-formyl-OEP (3) revealed, surprisingly, the formation of the cis-isomers 1,2-bis[5-(2,3,7,8,12,13,17,18-octaethylporphyrinato)metal(II)]-cis-ethene (4,5). This is in contrast to other reported reductive couplings of aldehydes and ketone which usually lead to the trans-product. The cis-isomer can undergo an acid catalyzed isomerization to the trans-isomer (6,7) which is also formed as a side product in the coupling reaction. An explanation for the formation of the cis-ethene-bridged bis-porphyrins may involve the strong aggregation tendencies of metalloporphyrins. These aggregation forces are evidenced in the crystal structures of the cis-dimers which exhibit considerable overlap of the p-systems and have a cofacial macrocycle arrangement with almost coplanar macrocycles. Besides the strong intramolecular aggregation effects, intermolecular p-stacking was observed in the structures of the Cu(II) (4) and Ni(II) (5) cis-dimers, the nickel(II) trans-isomer (7) and the related structure of bis[2-(3,7,8,12,13,17,18-heptaethylpophyrinato)nickel(II)]hydroxymethylene (13) which is obtained by low valent titanium coupling of Ni(II) 2-formyl-OEP (12). The synthesis of the cofacial cis-dimers is achieved in a single step from the corresponding formylporphyrins in high yields, thus presenting an improvement over other published procedures for synthesis of face-to-face bis-porphyrins. Crystal data: 4, Mo Ka (k = 0.71069 Ð) at 130 K, a = 19.208(7) Ð, b = 14.672(5) Ð, c = 24.170(7) Ð, b = 111.15(3)³, V = 6352(4) Ð3, Z = 4, space group P21/n, R = 0.056; 5, Mo Ka (k = 0.71069 Ð) at 130 K, a = 21.671(7) Ð, b = 14.079(5) Ð, c = 22.110(7) Ð, b = 108.86(2)³, V = 6384(3) Ð3, Z = 4, space group P21/c, R = 0.064; 7, Cu Ka (k = 1.54178 Ð) at 126 K, a = 11.163(3) Ð, b = 12.775(4) Ð, c = 14.554(3) Ð, a = 69.59(2)³, b = 81.15(2)³, c = 73.21(2)³, V = 1859(1) Ð3, Z = 1, space group P1, R = 0.072; 13, Mo Ka (k = 0.71069 Ð) at 130 K, a = 40.31(2) Ð, b = 14.997(7) Ð, c = 21.954(11) Ð, b = 108.6(4)³, V = 12579(10) Ð3, Z = 8, space group C2/c, R = 0.122.