3. Internal connections of the 4n π-electron system: Covalent bonds that connect atoms of the 4n π-electron system directly, so without an additional structural unit, can conceal the antiaromaticity / reduce the diradical character. This is particularly effective if it creates locally aromatic units (in which all π-electrons of these locally aromatic units also form part of the formal 4n π-electron system), as in biphenylene (Figure left). However, internal connections can considerably reduce the ability of the molecules to further conceal the antiaromaticity by conformational changes. Therefore, this structural motif is often found in combination with other structural motifs (especially when the internal connection does not create locally aromatic units), as in dibenzopentalene (Figure right). As a molecular design strategy, connecting the 4n π-electron system internally offers less flexibility, as no additional atoms are introduced. Substituents at the 4n π-electron system or heteroatoms in the 4n π-electron system can also have an impact on the level of concealment but are normally only found in combination with one or more of the structural motifs outlined above. For example, Ni(II) norcorrole features two nitrogen atoms in the formal 4n π-electron system, two locally aromatic subunits that share 4 π-electrons with the formal 4n π-electron system, as well as two vinylene substituents that bridge the π-electron system but do not create locally aromatic units (Figure right). The substitution of carbon atoms in the 4n π-electron system with nitrogen atoms, as in Ni(II) norcorroles and as studied computationally for pentalene, is considered to add to the level of concealment. While the structural motifs and molecular design strategies outlined above can certainly help to design molecules with concealed antiaromaticity, it should be noted that they may be oversimplifying in some cases: For Ni(II) norcorrole, computations suggest that an aromatic "halo" around the antiaromatic core may contribute to its stability. The computations also indicate that the actual delocalization pathway may differ from the formal 4n π-electron system in this molecule.