The Bl,CI,C2 plane forms an angle of 85" with the CI,B2,C14 plane. The duryl rings are twisted by 49 and 88" with respect to the BI,Cl,C2 and C2,B2,C1 planes. The C1C2 bond of 137 pm in the three-membered ring is considerably longer than the CC double bonds in cyclopropenes (130 pm ["]) but the same as that in the tri-fert-butylcyclopropenium
The BC bond lengths in the threemembered ring of 144 and 152 (av. 148) pm are markedly shorter than the value of 155 pm calculated for the BC single bond in borirane.I"l The bond lengths in the borirene indicate delocalization of the two A-electrons.
The BI,C2,B2 angle in 1 (146.3") is very similar to that calculated for 2, (145.7"),'51 unlike that for 2, (157.2°).151 The different BC bond lengths in the ring of 1 and the exocyclic BC bond length of 150 pm, which is very short for a BC single bond, are in good agreement with the data calculated for 2,, and therefore represent evidence for pronounced hyperconjugative interaction in orthogonal C-borylborirenes. Hyperconjugation is normally found only in charged systems."'71 The strong hyperconjugation in neutral C-borylborirenes is due to a combination of structural units which are most effective in this interaction, i.e., a strained three-membered ring (cf. 4["l) and atoms of low electronegativity (cf. 5;1'91 in 1 and 2 it is the boron atom with negative TI-charge).
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