When BCl_(3) is treated with water, it hydrolysesand forms [B(OH)_(4)]^(-) only whereasAlCl_(3)in acidifiedaqueous solutionforms [Al(H_(2)O_(6))]^(3+) ion. Explain what isthe hybridisationof boron and aluminium in these species ?

When BCl_(3) is treated with water, it hydrolysesand forms [B(OH)_(4)]

1.	When BCl_(3) is treated with water, it hydrolysesand forms [B(OH)_(4)]^(-) only whereasAlCl_(3)in acidifiedaqueous solutionforms [Al(H_(2)O_(6))]^(3+) ion. Explain what isthe hybridisationof boron and aluminium in these species ?
Answer» Solution :When `BCl_(3)` reacts with water, it first undergoeshydrolysis to form boric acid, `B(OH)_(3)`.Due to small size and high electronegativity of B, `B(OH)_(3)`polarizes `H_(2)O` moleculeacceptingan `OH^(-)` ion to form `[B(OH)_(4)]^(-)`speciesand releasinga proton : `BCl_(3) + 3H_(2)O rarr B(OH)_(3) + 3HCl : B(OH)_(3) + H_(2)O rarr [B(OH)_(4)]^(-) + H^(+)` Since B lies in the 2ndperiod,it has onlyibe s-and three p-orbitalsbut no d-orbitals .In other words, at the maximum, it canhave fourpairs ofelectrons in the valenceshell, ie., its maximumcoordinationnumberis In contrast, `AlCl_(3)` undergoes hydrolysis in acidfied aqueous solution to form `[Al(H_(2)O)_(6)]^(3+)` `AlCl_(3) + "water" overset("HCL")rarr[Al(H_(2)O)_(6)]^(3+) + 3Cl^(-) (aq)` This may be explainedas follows : (i) In acidicmedium, the conc.of `OH^(-)`IONS is much lower than that of`H^(+)` atomstherefore,`Al^(3+)` C ions coordinatewith `H_(2)O` moleculesand not with `OH^(-)` ions. (ii) Due to the presence of vacant d-orbital in `Al^(3+)`ions,ITCAN expand its coordinationnumber from 4 to 6and hence forms `[Al(H_(2)O)_(6)]^(3+)` in whichhybridizationof `Al` is `sp^(3)d^(2)`.