Title compd. (I) was prepd. by reductive coupling of tri-tert-butylcyclopropenylium tetrafluoroborate (II) with Li. Upon treatment with AgBF4 I does not undergo valence isomerization to the Dewar form of hexa-tert-butylbenzene but is reoxidized to substrate II. The photochem. behavior of RCON:NR, prepd. by treating II with Me3SiCLiN2, is also unusual. Irradn. at room temp. results in gas evolution, however, instead of R2CO, RCOCOR is isolated. R2CO is only formed if RCON:NR is irradiated in an org. matrix at -196 Deg.