please explain back bonding in boron tri halides

Boron atoms in BX3 has six electrons in the outermost shekll and thus it can accept a pair of electrons from a donor molecule like NH3 to complete its octet hence the lewis acid character of boron trihalides is found : BI3 > BBr3 > BCl3 > BF3 . However the order is reverse according to the electronegativity of the halogen. Fluorine being the most electronegative should create the greatest electron deficiency on boron and thus BF3 should accept electron pair from a donor very rapidly than in other boron trihalides but that is not true. The anomalous behaviour has been explained on the tendency of the halogen atom to BACK-DONATE its unutilised electrons to the vaccant p-orbitals of boron atom . In trifluoride each fluorine has completely filled with unutilised 2p orbitals while boron has a vaccant p-orbitals of boron. Now since both of these orbitals belong to the same energy level (2p) they can overlap effectively as a result of which fluorine electrons are transfered into the vaccant 2p-orbital of boron resulting in formation of an additional p(pi)-p(pi) bond. These type of bond formation is known as BACK BONDING or BACK DONATION. Thanks & Regards Akshay Meena, askIITians Faculty M.Tech, IIT Kharagpur

In boron tri floride ,boron has one vacant p-orbital .Each F atom has 2 fully filled orbitals left which is not used .F transfer 2 electrons to vacant 2p orbitals of boron ,thus forming pi-pi bonding this type of bond has some double bond character i.e dative bond {back bonding} All the bond length are the same and it is possible when double bond is delocalized. This bond reduces the electrons defficiency of boron atom .thus it`s lewis acid character decreases the tendency of forming back bonding is maximum in BF3 and decrease from BF3 to BI3