Transition metal systems atomic natural orbitals

On the other hand, the excited energy state diagrams of porphyrins with transition metals are rather complicated when the d orbitals of metal intervene between the n and n orbitals of the porphyrin system. The d d, n d, and d tf states are drawn over the normal porphyrin itstate. The relative positioning is complicatedly dependent on metal atoms, substituents, and axial ligands. Extended Huckel (EH) MO calculation has suggested that the lowest excited states should have a d d nature for Ni" [32-34], Co [36,37] n d... 

Summary Complicated nature of aromaticity in all transition metal cyclic systems can be understood more easily on simplified model cyclic triatomic and tetratomic systems as examples of cyclic systems composed out of odd or even number of atoms, respectively. Counting rules for a-, n-, 8-, and singlet/triplet coupled triatomic and tetratomic systems depend on the nature of atomic orbitals involved in the formation of corresponding bonding/antibonding molecular orbitals. 

---