The number of electrons in the π-system is also altered by forming the corresponding cations and anions. A [3]cumulene dianion retained its linear structure with co-planar end-groups in the crystal structure. Anions and strongly donor-substituted allenes may bend to accommodate the additional charge in the π-system. . We focus on cations and dications where linearity is preserved. With two electrons less, the dicationic structures change 90 • relative to the neutral species. Shown in Figure , the allene dication has 2 π-electrons and thus forms a planar system with Hückel topology. The [3]cumulene dication has perpendicular end-groups in accordance with a 4 π-electron Möbius system, and the diaminoacetylene dication has co-planar end-groups as its 6 π-electrons require a Hückel topology. Notably, the torsion energy profiles are interchanged between the neutral and dication species (cf. Figure and). Spiroconjugated molecules can also be depicted with a coarctate basis. There are 4 π-electrons in spiropentadiene and 8 in spirononatetraene, which fit their perpendicular configuration. However, the 6 π-electron [2,4]spiroheptatriene is perpendicular around the spiro-atom, and both neutral [3,3]spirosystems and the spiro[2,3]hexadienyl anion are unstable. Spirometallic 4n+2 π-systems do assume planarity, and the spiropentadiene dication is also planar. The torsion energy profiles of neutral and dication spiropentadiene are very similar to the equivalent allene species (Figure and), and we shall explore this connection for their cations.