STABILIZATION OF D3H PENTACOORDINATE CARBONIUM-IONS - LINEAR 3-CENTER-2-ELECTRON BONDS - IMPLICATIONS FOR ALIPHATIC ELECTROPHILIC SUBSTITUTION-REACTIONS

The structures of pentacoordinate carbon species, CH3M2 + (M = Li, BeH, Na, and MgH), were probed by ab initio molecular orbital calculations. Two general types of structures were considered, those with Cs symmetry which model electrophilic substitutions occurring with retention of configuration, and those with D3h symmetry modeling processes proceeding with inversion. All ions studied were indicated to be very stable toward dissociation; CH3Li2 + is known experimentally in the gas phase. In contrast to CH5 +, which prefers Cs structures to D3h, some CH3M2 + species favored D3h, geometries. At the RHF/6-31G* level the Cs-D3h energy differences follow: CH3Li2 +, 2.5; CH3(BeH)2 +, 5.7 kcal/mol. Electron correlation does not appear to change these differences appreciably. These results indicate that systems with three-center-two-electron bonds may favor linear over cyclic arrangements, depending on the atoms involved. Likewise, electrophilic aliphatic substitutions of this type can be expected to proceed either with inversion or with retention depending on the conditions; there may be no strong inherent preference for either stereochemical pathway. © 1979, American Chemical Society. All rights reserved.