What is it about?
Dodecasubstituted porphyrins are so-called highly substituted porphyrins which are often nonplanar, i.e. they do have a highly distorted macrocycle structure. Taking a large "sitting-atop" metal ion, such as thallium(III), results in a reflattening of the macrocycle. Thus, metal effects can counteract the macrocycle distortion imposed via steric hindrance of peripheral substituents.
The following have contributed to this page: Professor Mathias O. Senge