Question

All the boron trihalides except B may be prepared by direct reaction between the elements Boron trihalides consist of trigonal - planar BX₃ molecules Unlike the halides of the other elements in the group they are monomeric in the gas, liquid and Sol. id states, BF₃ and BCl₃ are gases, BBr₃ 3Iis a volatile liquid and B is a Sol. Id Boron trihalides are Lewis acids because they form simple Lewis complexes with suitable bases, as in the reaction : However, boron chlorides, bromides and iodides are susceptible (sensitive) to protoly sis by mild proton sources such as water, alcohols and even amines; for example BCl3 undergoes rapid hydrolysis: It is supposed that the first step in the above reaction is the formation of the complex which then eliminates HCl and reacts further with water
Which of the following is the correct prediction about observed B–X bond length, in BX₃ molecules ?

1. B–F bond length in BF₃ is found to be less than theoretical value because the electronegativity values of B(2.04) and F(4.0) suggest the bond to be ionic and hence the attraction between oppositely charged ions must decrease the bond length    
2. BF₃ and [BF₄ ] ^– have equal B–F bond length    
3. The decrease in the B–F bond length in BF₃ is due to delocalised bonding between vacant ‘2p’ orbital of B and filled ‘2p’ orbital of F    
4. The correct B–X bond length order is B–F > B–Cl > B–Br I> B–