For the M(III) ions of Al,
Cr, and Co, the metal can basically only move in and out of the
plane of the porphyrin. Dissociation of an arm or adoption of a cis
octahedral LM(X)(Y) geometry is impossible. The porphyrin
ligand requires the trans-LM(X)(Y) geometry. Any changes in
the substituents on the porphyrin occur at a significant distance
from the active metal center