CH 5-Type Cations

(CH)5+-Type Cations. The close relationship between carbocations and boranes led Williams1077 to suggest the square-pyramidal structure 615 for the (CH)5+ cation based on the square pyramidal structure of pentaborane. Stohrer and Hoffmann1078 subsequently came to the same conclusion concerning the preferred square-pyramidal structure for the (CH)5+ cation using extended Hiickel MO calculations. 

Cation 615 with C4v symmetry can be viewed1078 as square cyclobutadiene capped  

Attempts have been made to observe1084 the assumed interconversion of 625 to 626 in fluorenyl cations. Such intramolecular interconversion through the capped pyramidal ion 627 was not observed [Eq. (3.150)]. MINDO/3 calculations1084 on the isomeric structures of cyclopentadienyl, indenyl, and fluorenyl cations indicated strongly decreasing relative stabilities of pyramidal forms due to bezannulation. 

Insertion of a methylene group into the four-membered ring of the pyramidal (CH)5+ cation 615 would give rise to the homo derivative 628. As discussed earlier,41 1037-1041 hexamethylbicyclo[2.1.1]hexenyl cation is best represented as a bishomocyclopropenyl cation 586 and not pyramidal-type ion 629. 

---