ELECTRON DEFICIENT SPECIES

Important examples of electron deficient species are carbocations [CR3]+ and BF3 (and BCl3 and BBr3). Carbocations are generated by loss of two electrons from the carbanion, [CR3]–, so the C formal charge increases from –1 to +1. They can also be recognized as being electron deficient because the formal charge based on the number of bonds will appear to be –1 with no obvious way to increase the charge to +1. BF3 can in fact be drawn as three non-electron-deficient resonance structures as below. The short B–F bond allows for good π-π overlap, and F atoms are good π donors.  BF3 is also significantly less reactive than BCl3 and BBr3 where the long B–Cl and B–Br bonds limit stabilization by π bonding.

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BF3 is still reactive and donor molecules readily form complexes such as BF3NH3 via dative bonds. Note that a dative bond is in fact just a covalent bond with a shared electron pair. The bonding electrons do not remember where they came from! Thus, the zwitterionic (a species with both negative and positive formal charges) structure on the right is also valid and is an acceptable, albeit uncommon, representation. A similar situation arises with species such as amine oxides, where it is actually more common to see the structure on the right. Interestingly, Lewis himself essentially described dative bonds as zwitterionic covalent bonds.