Structural investigations of copper and nickel complexes of dodecasubstituted porphyrins bearing aryl groups at the meso positions and propane rings at the pyrrole β positions reveal considerable differences in their macrocycle conformations. While the nickel complex NiTC5T(3,4,5-OMeP)P was found to exhibit a nonplanar conformation which is considerably more planar than that of other dodecasubstituted porphyrins, the corresponding copper complex CuTC5T(3,4,5-OMeP)P was planar. CuTC5T(3,4,5-OMeP)P thus represents the first example of a completely planar dodecasubstituted porphyrin. The crystal structures of both porphyrins reveal that the Cb-Cb-CH2 angle is 13° smaller than in OEP derivatives. This change, which moves the methylene and aryl substituents further apart, effectively removes the steric repulsion responsible for the very nonplanar conformations observed for other dodecasubstituted porphyrins. Molecular mechanics calculations using a porphyrin force field correctly predict a planar macrocycle conformation. The possible reasons for the discrepancy between the observed moderately nonplanar structure and the calculated planar structure for NiTC5T(3,4,5-OMeP)P are discussed. The usefulness of spectroscopic probes (NMR, resonance Raman, electronic absorption) in predicting the planarity of dodecasubstituted porphyrins is also examined. The identification of a planar dodecasubstituted porphyrin further indicates the flexibility of the tetrapyrrole macrocycle and has implications for the study of nonplanarity in synthetic porphyrins and metallotetrapyrrole containing biomolecules. Crystal data: NiTC5T(3,4,5-OMeP)P·2CH2Cl2 (C68H68N4NiO12·2CH 2Cl2), triclinic, P1, a = 14.492(8) Å, b = 14.453(8) Å, c = 17.689(10) Å, α = 103.11(5)°, β = 113.58(4)°, γ = 89.93(5)°, V = 3290(3) Å3, Z = 2, λ(Mo Kα) = 0.7107 Å, T = 130 K, R = 0.06 for 7657 reflections with I > 2.5σ(I); CuTC5T(3,4,5-OMeP)P·CH2Cl2 (C68H68N4CuO12·CH 2Cl2), monoclinic, P21/c, a = 14.459(4) Å, b = 15.302(6) Å, c = 14.879(4) Å, β = 100.68(4)°, V = 3235(2) Å3, Z = 2, λ(Mo Kα) = 0.7107 Å, T = 130 K, R = 0.106 for 4222 reflections with I > 2.0 σ(I).