LEWIS ACIDITY OF BORON TRIHALIDES

An electron deficient specie is termed as a lewis acid (electrophile).Going by the description,the trihalides of Boron is electron deficient (due to the presence of an empty pz orbital in boron).A Lewis Base(Ligand) will always attack an electron deficient center.The availability of this center to Ligand attack varies in BX3,where X=F,Cl,Br and I.Several factors accounts for this,let's look at some few ones. Steric factor and Electronegativity argued that BF3>BCl3>BBr3>BI3. The more the electronegativity the more the deficiency of electrons on B atom,the more the acidity. Steric hinderance,the much bulkier the halide the much hinderance it offers to attacking nucleophile. But this failed because it can't account for the difference in bond length of B-L in the adduct formed. Pyramidation and Pie-bonding succeed in explaining the trend.Pie-bonding results when the lone pairs electron on the halide are been donated to the empty pz orbital of B(check the image above) decreasing its Lewis Acidity and this is more effective in F>Cl... That means the acidic trend is of the decreasing order BI3>BBr3>BCl3>BF3 Pyramidation also agrees with this trend BF3 is highly pyramidized than others which also decreases its acidity.