In a push-pull effect, indoliums acquire this property when covalently bonded gold-tri·phenyl·phosphine provides a source of hyperconjugation. The gas-phase lifetime of the tetra·aluminum di·an·ion is attributed to this property, which is inferred using the GIMIC (“gimmick”) method for inorganic complexes like closo deltrahedral boranes or reduced buckyballs. Osma·penta·lynes have this property because the metal d-orbitals twist (-5[1])so the nodal plane traces out a (*) Möbius strip. (10[1])Ghost atoms at the (10[1])center of molecules with this property experience a downfield chemical shift caused by (10[1])a diatropic ring current. (10[3]-5[1])Cyclo·buta·diene switches to having this property (-5[1])when it coordinates a metal and gains two electrons, bringing its (-5[1])pi count up to six. For 10 points, name this property that’s predicted for planar, conjugated hydrocarbons by Hückel’s rule. ■END■ (10[3]0[1])