New in the chemical zoo, well at least computational: the B18 dianion as reported by Moreno et al. here. This anion is predicted to have a pentagonal six-boron core surrounded by a bowl-shaped B12 ring. The pentagon can move freely inside the ring as in a Wankel engine and the molecule is therefore fluxional although the imaginary temperature in the simulation had to be cranked up to 900 Kelvin. The inner core will not actually fall out because of a continuous process of bond making and bond breaking. Based on the total number of pi electrons the molecule should be antiaromatic but in this particular configuration the core is again aromatic.