Why BI3 is more acidic then BF3?

Answer 1

) Boron atom, in BX3, has six electrons in the outermost orbit and thus it can accept a pair of electrons form a donor molecule like NH3 to complete its octet. Hence boron halides act as very efficient Lewis acids. The relative Lewis acid character of boron trihalides is found to obey the order ; BI3>BBr3>BCl3>BF3.

However, the above order is just the reverse of normally expected order on the basis relative electronegativities of the halogens. Fluorine, being the most electronegative, should create the greatest electron deficiency on boron and thus B in BF3 should accept electron pair from a donor very rapidly than in other boron trihalides. But this is not true.

This anomalous behavior has been explained on the basis of the relative tendency of the halogen atom to back-donate its unutilised electrons to the vacant p orbitals of boron atom. In boron trifluoride, each fluorine has completely filled unutilised 2p orbitals while boron has a vacant 2p orbital. Now since both of these orbitals belong to same energy level (2p) they can overlap effectively as a result of which fluorine electrons are transferred into the vacant 2p orbital of boron resulting in the formation of an additional pπ-pπ bond. This type of bond formation is known as back bonding or back donation. Thus the B- F bond has some double bond character. Back bonding may take place between boron and of the three fluorine atoms and thus boron trifluoride is regarded as a resonance hybrid of some structures.

Resonance in boron trifluoride is also evidenced by the fact that the three boron-fluorine bonds are indentical and are shorter than the usual single boron-fluorine bond As a result of back bonding, the electron deficiency of boron is reduced and hence Lewis acid nature is decreased. The tendency for the formation of back bonding (pπ-pπ bond) is maximum in BF3 and decreases very rapidly from BF3 to BI3This is probably due to the fact that overlapping of the vacant 2p orbitals of boron cannot take place easily with the p-orbitals of high energy levels (3p in Cl, 4p in Br and 5p in iodine). Thus BI3Br3 and BCl3are stronger Lewis acids than the BF3.

Answer 2

In BF3, the boron atom lacks a complete octet and can only accept electrons, making it a Lewis acid. In BI3, the larger size of the iodine atom allows for greater electron delocalization, stabilizing the conjugate base and making it less reactive. Therefore, BI3 is more acidic than BF3.