Crystallographic evidence for the diene character of C 2 B 10 H 10 C 4 H 4 (‘benzocarbonae’) and a Diels–Alder reaction of its anionic nido-analogue, [C 2 B 9 H 10 C 4 H 4 ] – : crystal structures of C 2 B 10 H 10 C 4 H 4 and C 2 B 10 H 10 C 4 H 6

Single-crystal X-ray diffraction studies on [C2B10H10C4H4](‘benezocarborane’) and C2B10H10C4H6(‘dihydrobenzocarborane’) show that the C6 rings of each posses localised double bonds and this has been confirmed by bond-order calculations: the nido-anion. [C2B9H10C4H4]– synthesised from C2B10H10C4H4, with maleic anhydride forms a Diels-Alder adduct.