text: Porphyrin complexes consist of a square planar MN core.  The periphery of the porphyrins, consisting of sp-hybridized carbons, generally display only small deviations from planarity.  Additionally, the metal is often not centered in the N plane.  
  Large metals such as zirconium, tantalum, and molybdenum tend to bind two porphyrin ligands.  Some [M(OEP)] feature a multiple bonds between the metals.