1,3,5,7-Tetra-tert-butyl-4-aza- and 1,3,5,7-Tetra-tert-butyl-4-phospha-s-indacene

s-Indacenes reveal the limits of the Hückel rules: These 12π systems are formally antiaromatic, but the results of more recent quantum mechanical calculations indicate π-electron delocalization. The synthesis and spectroscopic investigation of the non-electronically stabilized 4-hetero-s-indacenes 1 and 2 have now been achieved for the first time.