Two types of halides are available for this group. One is MX3 and another is MX5 type.
MX3: NCl3, PCl3, AsCl3, SbCl3 and BiCl3
MX5: PCl5, AsCl5, SbCl5
(a) Tri halides
(i) All trihalides are covalent except BiF3 which is ionic.
(ii) Like hydrides these trihalides have pyramidal structure and a central atom is sp3 hybridized.
(iii) These trihalides can be easily hydrolyses by water except NX3.
PCl3 + 3H2O → H3PO3 + 3HCl
AsCl3 + H2O → As2 O3 + 6HCl
SbCl3 + H2O → SbOCl + 2HCl
BiCl3 + H2O → BiOCl + 2HCl
(iv) The trihalides of P, As and Sb acts as Lewis acids and combine with Lewis bases.
PF3 + F2 → PF5
SbF3 + 2F– → [SbF5]2-
(b) Penta halides
In this central atom attains sp3d hybridization and forms five covalent bonds with five chlorine atoms.
Penta halides have less thermal stability as compared to trihalides. All penta halides act as Lewis acids.
PCl5 + Cl– → PCl 6 –
SbCl3 Cl – → [SbCl4]–
PCl5exists 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.
Illustration 1. Are all the five bonds of PCl5 equivalent? Justify your answer.
Solution: PCl5 has trigonal bipyramidal 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.
Illustration 2. Among the trihalides of nitrogen, which one is the least basic?
(A) NF3 (B) NCl3
(C) NBr3 (D) NI3
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