Chemistry - Complete Pblock- Question with Solution | TestHub

In$\mathrm{B}\mathrm{C}{\mathrm{l}}_3,\mathrm{p}\mathrm{\pi }-\mathrm{p}\mathrm{\pi }$back-donation occurs between the vacant 2p-orbital of 'B' and fully-filled 3p-orbital of Cl of a$\mathrm{B}-\mathrm{C}\mathrm{l}$bond. This$\mathrm{p}\mathrm{\pi }-\mathrm{p}\mathrm{\pi }$overlapping is not much effective, because of dissimilar sizes of 2p and 3p-orbitals. But in$\mathrm{A}\mathrm{l}\mathrm{C}{\mathrm{l}}_3$,$\mathrm{p}\mathrm{\pi }-\mathrm{p}\mathrm{\pi }$overlapping is more effective, because p-orbitals of Al and Cl belongs to the same energy levels and hence, they are of similar sizes. As a result of back-donation, the electron-deficiency of Al atom gets more compensated as compared with 'B' in$\mathrm{B}\mathrm{C}{\mathrm{l}}_3$and therefore, Lewis acid character of$\mathrm{A}\mathrm{l}\mathrm{C}{\mathrm{l}}_3$is lesser than that of$\mathrm{B}\mathrm{C}{\mathrm{l}}_3$.