p-pi back bonding

BF₃ molecule has three B-F sigma bonds each of which is formed by the overlap between the singly-filled sp² hybrid orbital on Boron atom and singly filled 2px orbital on Fluorine atom (2s²2px¹y²z², x,y,z symmetrical hence x to be singly filled). Due to sp² hybridization and no lone pair of electrons on B-atom, BF₃ molecule has the expected trigonal planer shape. Each of the three F-B-F bond angles is equal to 120⁰. sp² hybridization given above indicates that the vacant 2pz on B-atom does not participate in sp² hybridization process and hence remains unused. Completely filled 2pz on any of the three F-atoms donates its electron pair to the vacant and unused 2pz orbital on B-atom and gives rise to formation of F(2pz²) ® b(2pz⁰) coordinate pi bond. Thus we find that an additional F®b pi bond is formed by the overlap of two p-orbitals, it is (p-p)pi or (p-pi-p-pi)bond. The formation of F®B pi bond is called  back bonding. This back bonding creates eight electrons ( octet of electrons or inert gas configuration) round B-atom in BF₃ molecule. In this way B-atom which is surrounded by six electrons in BF₃ molecule gets surrounded by eight electrons and thus compensates its deficiency of two electrons. Since 2pz orbital on B-atom is filled by the pair of electrons donated by F atom, the electron deficiency of B-atom gets compensated and therefore, the Lewis acid character of BF₃ molecule decreases.

All B-F bond lengths in BF₃ molecule are equal (=130 pm) which are shorter than the sum of covalent radii of B & F atoms ( B=80 pm, F=72 pm). Since all the three B-F bond lengths are equal, the true structure of BF₃ molecule can be supposed to be a resonance hybrid of three structures. The p-pi-p-pi bond is not localized between any two particular B & F atoms, rather it is spread over the molecule. Thus pi bond is a delocalised pi bond.

In case of BCl₃ and BBr₃, 3p(Cl)®2p(B) and 4p(Br)®2p(b) respectively, would not be very effective ,since there is an appreciable difference in energy levels of 3p, 4p and 2p orbitals.

As a consequence of back bonding the Lewis acid character ( to accept pair of electrons from Lewis bases) of BX₃ molecule is in the order

BF₃ < BCl₃ < BBr₃ < BI₃