A @C00817@ (real or hypothetical) in which there are two (or more) carbon atoms that could in alternative @L03513@ be designated as @C00811@ but which is instead represented by a structure in which a group (a hydrogen atom or a hydrocarbon residue, possibly with substituents in non-involved positions) bridges these potential carbenium centres. One may distinguish 'electron-sufficient bridged carbocations' and 'electron-deficient bridged carbocations'. Examples of the former are phenyl-bridged ions (for which the @T06516@ 'phenonium ion' has been used), such as (A). These ions are straightforwardly classified as @C00812@. The latter type of ion necessarily involves three-centre bonding. Structures (C) and (D) contain five-coordinate carbon atoms. The 'hydrogen-bridged @C00817@' (B) contains a two-coordinate hydrogen atom. @HT07051@, which includes two-@C01329@ for hydrogen and five- but also higher @C01329@ for carbon is generally observed in bridged carbocations.
neighbouring group participation