Dodecaalkyl/aryl-substituted porphyrins have been shown to exhibit severely non-planar macrocycle conformations with displacement of the C_b-atoms of up to 1.2 Å. 2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetranitroporphyrin (H₂oetnp) presents an easily available model compound for studies on the conformational flexibility of tetrapyrroles. In contrast to most known structures of dodecasubstituted porphyrins, which exhibit severely non-planar macrocycle conformations, the thallium(III) complex [Tl(oetnp)Cl], shows only a moderately distorted macrocycle with an average displacement of the C_b-atoms of 0.19 Å. This is the first example of a dodecasubstituted porphyrin where the effect of the large main-group metal ion counterbalances the conformational distortion induced by steric strain of the peripheral substituents. Out-of-plane relief of the steric strain is prevented by the thallium atom; rather, inplane distortion of the macrocycle, with widened C_a–C_m–C_a angles, is observed. Structural investigations of the free-base porphyrin and its nickel(II) complex show very non-planar conformations and severe saddle-shaped distortion indicated by displacements of the C_b-atoms of up to 1.05 Å. The mode and degree of distortion are similar to those described for dodecaalkyl/aryl-porphyrins indicating that the oetnp ligand is non-planar and that the much smaller degree of distortion observed in the thallium derivative is due to a metal effect. This striking difference of conformational distortion on dependence of the central metal is the first example where such large differences in non-planarity of a given tetrapyrrole ligand have been characterized by structural means. Crystal data: H₂oetnp, orthorhombic, Pna2₁, a= 11.114(4), b= 12.409(5), c= 25.905(11)Å, Z= 4, R= 0.046 for 2783 reflections with l > 2.5σ(l); [Ni(oetnp)], triclinic, P, a= 10.610(2), b= 13.678(5), c= 24.884(8)Å, α= 95.89(3), β= 93.19(2), γ= 92.70(2)°, Z= 4 (2 independent molecules), R= 0.063 for 8649 reflections with l > 2.0σ(l); [Tl(oetnp)Cl], triclinic, P, a= 13.968(4), b= 14.091(3), c= 18.737(4)Å, α= 94.80(2), β= 99.94(2), γ= 90.43(2)°, Z= 4 (2 independent molecules), R= 0.079 for 7640 reflections with l > 1.5σ(l).