how can we compare the bond angles in BF3 BCl3 BBr3 BI3

how can we compare the bond angles in BF3 BCl3 BBr3 BI3


1 Answers

Deepak Kumar Shringi
askIITians Faculty 4407 Points
4 years ago

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.

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