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Halides of Nitrogen family- 15th Group:

Two types of halides are available for this group. One is MX3 and another is MX5 type.
(A) MX3 Type or Trihalides:
MX3:

NCl3, PCl3, AsCl3, SbCl3 and BiCl3
Characteristic of Tri halides:
(1) All possible trihalides of all the elements of nitrogen family are known except NCl3, NBr3, and NI3.
(2) These are (NCl3, NBr3, and NI3) unstable due to the low polarity of the (N – X) bond. And Weakness of (N - X) bond due to large size difference.
(3)  All trihalides are covalent except BiF3 which is ionic.
(4)  Like hydrides these trihalides have pyramidal structure and a central atom is sp3 hybridized.

Structure of PCl3:
(5)  These trihalides can be easily hydrolyses by water except NX3.

SN
Halides
 Hydrolysis

NF3
Do not Hydrolysed at room temperature
1
NX3 (X= Cl, Br, I)
NX3 +3H2O à NH3 + 3HOX          (At RT)
2
PX3 (X= F Cl, Br, I)
PCl3 +  3H2O à H3PO3 + 3HCl
2
AsCl3
AsCl3  + H2O à AsO3 + 6HCl    or
AsCl3  + 3H2O à H3AsO3 + 3HCl
4
SbCl3
SbCl3 + H2O à SbOCl (White turbidity) + 2HCl
5
BiCl3
BiCl3 + H2O à BiOCl  (White turbidity) + 2HCl

Imp note:
(i) NF3 Hydrolysed only at high temperature (300 0 C) and give N2O3 which is unstable and dissociate to produced NO and NO3 gas.
                           2NF3 + 3H2O  à N2O3 +6HF à NO (gas) + NO2 (gas) + 6HF
(ii)When an aq. solution of BiCl3 is prepared, after some time turbidity appears which is milky in appearance and finally a white ppt is formed due to formation of BiOCl.
(6)  The trihalides of P, As and Sb acts as Lewis acids and combine with Lewis bases.
PF3 +F2  
à
PF5
SbF3 +2F
à
[SbF5]2-

 (7) Lewis Base Order of trihalides follows as
        NF3 < NCl3 <NBr3 < NI3

 Imp note: As electronegativity of halides decrease and availability of loan pair over nitrogen atom increases hence Lewis Base character increase
(B) MX5 Type or Pentahalides:
MX5:
        PCl5, AsCl5, SbCl5
Characteristic of Pentahalides:
(1) Except N and Bi all form Pentahalides, N does not form due to absence of d-orbital while Bi does not form due to inert pair effect.
(2) Phosphorous form Pentahalides of all halogens.
(3) Bismuth forms only BiF5.
(4) As and Sb form Pentafluorides and pentachlorides only.
(5) The central atom of Pentahalides attains sp3d hybridization and forms five covalent bonds with five chlorine atoms.
Structure of  PCl5  :
(6) Penta halides have less thermal stability as compared to trihalides. All Penta halides act as Lewis acids.
PF5 +F-   
à
[PF6]-
SbCl3 +Cl- 
à
[SbCl4]-

(7) PCl5 acts as an effective chlorinating Agent so it decomposes into PCl5 à PCl3 + Cl2
(8) PCl5 exists as molecule in gaseous state but in solid it exists as [PCl4]+ [PCl6]- and is ionic in nature. PBr5, PI5 also exist in the ionic form in solid state.
SN
Halides(Gaseous state)
Solid State
1
PCl5
[PCl4]+[PCl6]- [sp3 and sp3d]
2
PBr5
[PBr4]+[Br]-
3
PI5
[PI4]+[I]-

Structure of PCl5  in solid state:
ILLUSTRATIVE EXAMPLE (1): Are all the five bonds of PCl5 equivalent? Justify your answer.
SOLUTION: PCl5 has trigonal bipiramidal structure. It has three equivalent equatorial and two equivalent axial P – Cl bonds. However, due to greater bond pair – bond pair repulsions, the axial P – Cl bonds are longer and hence different from the three equatorial bonds.

ILLUSTRATIVE EXAMPLE (2): Phosphorus can form PCl5 but nitrogen can not form NCl5 why?
SOLUTION: Phosphorus forms PCl5 due to availability of vacant d – orbital though white P can extend its oxidation state but this is not applicable in case of N due to unavailability of d – orbitals.

ILLUSTRATIVE EXAMPLE (3): Why NCl3 can not be hydrolysed?
SOLUTION: Due to unavailability of vacant d – orbitals.

EXERCISE (1): Among the trihalides of nitrogen, which one is the least basic?
                        (A) NF3                                                 (B) NCl3
                        (C) NBr3                                                (D) NI3
EXERCISE (2): Which of the following halides is most acidic?
                        (A) PCl3                                                (B) SbCl­3
                        (C) BiCl3                                               (D) CCl4
EXERCISE (3): Which of the following is not hydrolysed?
                         (A) AsCl3                                              (B) PF3
                         (C) SbCl3                                               (D) NF3

SN
EXERCISE
ANSWER KEY
1
1
(A)
2
2
(A)
3
3
(D)


           


Related Questions:



Trisilyl amine, N(SiH3)3 is planar whereas trimethyl amines N(CH3)3 is pyramidal. Explain why?.















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