The low-valent titanium induced reductive coupling of metal(II) 5-formyl-octaethylporphyrin (OEP) (2, 3) leads to the formation of ethene-bridged bisporphyrins. 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 bisporphyrins 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 pi-systems and have a cofacial macrocycle arrangement with almost coplanar macrocycles. Besides the strong intramolecular aggregation effects, intermolecular pi-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)]hydroxymethane (13) which is obtained by low valent titanium coupling of Ni(II) 2-formyl-OEP (12).