Square-pyramidal carbocation

A series of hypercoordinated square-pyramidal carbocations were optimized at the MP2/6-31G(d) level and the 13C NMR chemical shifts of the carbocations were calculated using IGLO-HF and GIAO-MP2 methods.86... 

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. 

The IR spectrum of the pentachlorocyclopentadi-enyl cation C5CI5+ (31) has been measured in an SbFs matrix, and this cation is determined to have D h symmetry by DFT calculations. Substituted cyclopentadienyl cations in the gas-phase undergo carbon skeletal rearrangements and loss of alkyne units that can be explained by the formation of carbocations with a square pyramidal structure (Figure 6). /) This strucTure was initially estimated to be more stable than planar structures, but later calculations confirm the latter are more stable. Cyclopentadienyl cations with several strongly electron-donating substituents have been obtained as stable salts. ... 

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