HOMOs and LUMOs in Reactions

The calculated lowest unoccupied molecular orbital (LUMO) for BF3 is shown by solid red and blue lobes. Most of the volume represented by the LUMO corresponds to the empty p orbital in the sp -hybridized state of BF3 (located perpendicular to the plane of the atoms). This orbital is where electron density fills (bonding occurs) when BF3 is attacked by NH3. The van der Waals surface electron density of BF3 is indicated by the mesh. As the structure shows, the LUMO extends beyond the electron density surface, and hence it is easily accessible for reaction. 

The highest occupied molecular orbital (HOMO) of ammonia, where the nonbonding pair resides, is shown by red and blue lobes in its structure. When the reaction occurs, the electron density from the HOMO of ammonia is transferred to the LUMO of boron trifluoride. This interaction 

In this way they are like BF3 and AICI3. Most carbocations are also short-lived and highly reactive. They occur as intermediates in some organic reactions. Carbocations react rapidly with Lewis bases—with molecules or ions that can donate the electron pair that they need to achieve a stable octet of electrons (i.e., the electronic configuration of a noble gas)  

Carbocation Anion (a Lewis acid) (a Lewis base) 

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