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Long answer questions
1. What are polyhalides? Give its classification. Discuss the structure of ICI, . (W-2005)
shastrictures of ICI.- and BrF.. (S-2006)
Answers
Explanation:
Polyhalogen ions are a group of polyatomic cations and anions containing halogens only. The ions can be classified into two classes, isopolyhalogen ions which contain one type of halogen only, and heteropolyhalogen ions with more than one type of halogen
Numerous polyhalogen ions have been found, with their salts isolated in the solid state and structurally characterized. The following tables summarize the species found so far.[1][2][3][4][5]
Isopolyhalogen cations
Diatomic species *[Cl2]+, [Br2]+, [I2]+
Triatomic species [Cl3]+, [Br3]+, [I3]+
Tetraatomic species [Cl4]+, [I4]2+
Pentaatomic species [Br5]+, [I5]+
Heptaatomic species †[I7]+
Higher species [I15]3+
* [Cl2]+ can only exist as [Cl2O2]2+ at low temperatures, a charge-transfer complex from O2 to [Cl2]+.[2] Free [Cl2]+ is only known from its electronic band spectrum obtained in a low-pressure discharge tube.[3]
† The existence of [I7]+ is possible but still uncertain.[1]
Heteropolyhalogen cations
Triatomic species [ClF2]+, [Cl2F]+, [BrF2]+, [IF2]+, [ICl2]+, [IBrCl]+, [IBr2]+, [I2Cl]+, [I2Br]+
Pentaatomic species [ClF4]+, [BrF4]+, [IF4]+, [I3Cl2]+
Heptaatomic species [ClF6]+, [BrF6]+, [IF6]+
Isopolyhalogen anions
Triatomic species [Cl3]−, [Br3]−, [I3]−
Tetraatomic species [Br4]2−, [I4]2−
Pentaatomic species [I5]−
Heptaatomic species [I7]−
Octaatomic species [Br8]2−, [I8]2−
Higher species [I9]−, [I10]2−, [I10]4−, [I11]−, [I12]2−, [I13]3−, [I16]2−, [I22]4−, [I26]3−, [I26]4−, [I28]4−, [I29]3−
Heteropolyhalogen anions
Triatomic species [ClF2]−, [BrF2]−, [BrCl2]−, [IF2]−, [ICl2]−, [IBrF]−, [IBrCl]−, [IBr2]−, [I2Cl]−, [I2Br]−, [AtBrCl]−, [AtBr2]−, [AtICl]−, [AtIBr]−, [AtI2]−
Pentaatomic species [ClF4]−, [BrF4]−, [IF4]−, [ICl3F]−, [ICl4]−, [IBrCl3]−, [I2Cl3]−, [I2BrCl2]−, [I2Br2Cl]−, [I2Br3]−,[I4Cl]−, [I4Br]−
Hexaatomic species [IF5]2−
Heptaatomic species [ClF6]−, [BrF6]−, [IF6]−, [I3Br4]−
Nonaatomic species [IF8]−
The bonding in polyhalogen ions mostly invoke the predominant use of p-orbitals. Significant d-orbital participation in the bonding is improbable as much promotional energy will be required, while scant s-orbital participation is expected in iodine-containing species due to the inert pair effect, suggested by data from Mössbauer spectroscopy. However, no bonding model has been capable of reproducing such wide range of bond lengths and angles observed so far.[3]
As expected from the fact that an electron is removed from the antibonding orbital when X2 is ionized to [X2]+, the bond order as well as the bond strength in [X2]+ gets higher, consequently the interatomic distances in the molecular ion is less than those in X2.
Linear or nearly linear triatomic polyhalides have weaker and longer bonds compared with that in the corresponding diatomic interhalogen or halogen, consistent with the additional repulsion between atoms as the halide ion is added to the neutral molecule. Another model involving the use of resonance theory exists, for example, [ICl2]− can be viewed as the resonance hybrid of the following canonical forms:
Canonical forms of (ICl2)-.png
Evidence supporting this theory comes from the bond lengths (255 pm in [ICl2]− and 232 pm in ICl(g)) and bond stretching wavenumbers (267 and 222 cm−1 for symmetric and asymmetric stretching in [ICl2]− compared with 384 cm−1 in ICl), which suggests a bond order of about
1
/
2
for each I–Cl bonds in [ICl2]−, consistent with the interpretation using the resonance theory. Other triatomic species [XY2]− can be similarly interpreted.[2]