text: Aromaticity is found in ions as well: the cyclopropenyl cation (2e system), the cyclopentadienyl anion (6e system), the tropylium ion (6e), and the cyclooctatetraene dianion (10e). Aromatic properties have been attributed to non-benzenoid compounds such as tropone. Aromatic properties are tested to the limit in a class of compounds called cyclophanes.
  A special case of aromaticity is found in homoaromaticity where conjugation is interrupted by a single sp³ hybridized carbon atom.
  When carbon in benzene is replaced by other elements in borabenzene, silabenzene, germanabenzene, stannabenzene, phosphorine or pyrylium salts the aromaticity is still retained. Aromaticity also occurs in compounds that are not carbon-based at all.  Inorganic 6-membered-ring compounds analogous to benzene have been synthesized.  Hexasilabenzene (SiH) and borazine (BNH) are structurally analogous to benzene, with the carbon atoms replaced by another element or elements.  In borazine, the boron and nitrogen atoms alternate around the ring. Quite recently, the aromaticity of planar Si rings occurring in the Zintl phase LiSi was experimentally evidenced by Li solid state NMR.
  Metal aromaticity is believed to exist in certain metal clusters of aluminium.
  Möbius aromaticity occurs when a cyclic system of molecular orbitals, formed from  p atomic orbitals and populated in a closed shell by 4n (n is an integer) electrons, is given a single half-twist to correspond to a Möbius strip. A π system with 4n electrons in a flat (non-twisted) ring would be anti-aromatic, and therefore highly unstable, due to the symmetry of the combinations of p atomic orbitals. By twisting the ring, the symmetry of the system changes and becomes allowed (see also Möbius–Hückel concept for details). Because the twist can be left-handed or  right-handed, the resulting  Möbius aromatics are dissymmetric or chiral. 
  As of 2012, there is no proof that a Möbius aromatic molecule was synthesized.
  Aromatics with two half-twists corresponding to the paradromic topologies were first suggested by  Johann Listing. In carbo-benzene the ring bonds are extended with alkyne and allene groups.