The cation 2 undergoes a number of other interesting reactions (Scheme 2). Thus, treatment with a slight excess of sBu,AIH (CH,C1,, -78°C) affords a 2:3 mixture of the starting material and the q4-buta-l ,3-diene complex 5, which could be separated by column chromatography (A1,0, grade 11, benzene/hexane, 25 "C). This finding can be rationalized in terms of a competitive reaction involving delivery of "H@" to the carbenoid or a-carbon of 2 and proton abstraction from the &-carbon. In contrast, reaction of 2 (CH,CI,, -78 "C) with EtC(CH,O),P does not lead to nucleophilic attack on the cx-carbon, but instead deprotonation occurs and the triphenylphosphane is replaced by the cage phosphite to form (column chromatography, silica, MeCN, -30°C) the pink solid 6. Although 2 does not react with carbon monoxide, treatment (CH,CI,/THF, -78 "C to + 25 "C and chromatography on silica/MeCN) with a two-fold excess of the isocyanides RNC (R = 2,6-Me2C,H,, tBu) resulted in the formation of red-orange 7 and yellow 8 respectively as slightly air-sensitive solids. Compound 7 was identified as the hexafluoroantimonate salt by comparison of the 'H-NMR and IR spectra with those of the fluoroborate species described by Yamazaki." 71 The corresponding spectroscopic data"'] for 8 suggested that an unexpected reaction had occurred involving loss of the tBu substituent, and this has now been confirmed by single crystal X-ray crystallography. Such a fragmentation reaction has not been previously obtained.