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Question: Which of the following gives the maximum number of isomers?...

Which of the following gives the maximum number of isomers?

A

[Co(NH3)4Cl2]

B

[Ni(en)(NH3)4]2+

C

[Ni(C2O4)(en)2]

D

[Cr(SCN)2(NH3)4]+

Answer

D

Explanation

Solution

Let's analyze each complex for possible isomers:

(A) [Co(NH3)4Cl2] This is an [MA4B2] type complex.

  • Geometrical Isomerism: It can exhibit cis and trans isomers.
    • cis-[Co(NH3)4Cl2]
    • trans-[Co(NH3)4Cl2]
  • Optical Isomerism: Neither the cis nor the trans form is chiral (they possess planes of symmetry).
  • Total Isomers: 2 (geometrical isomers).

(B) [Ni(en)(NH3)4]2+ This is an [M(AA)B4] type complex, where en (ethylenediamine) is a bidentate ligand. The coordination number is 6.

  • Geometrical Isomerism: Due to the presence of four identical NH3 ligands and one bidentate en ligand, there is only one possible arrangement.
  • Optical Isomerism: The complex possesses planes of symmetry.
  • Total Isomers: 1.

(C) [Ni(C2O4)(en)2] This is an [M(AA)(BB)2] type complex, where C2O4 (oxalate) and en (ethylenediamine) are both bidentate ligands. The coordination number is 6.

  • Geometrical Isomerism: It can exhibit cis and trans isomers.
    • cis-[Ni(C2O4)(en)2] (or cis-[Ni(en)2(C2O4)])
    • trans-[Ni(C2O4)(en)2] (or trans-[Ni(en)2(C2O4)])
  • Optical Isomerism:
    • The trans isomer has a plane of symmetry and is achiral. (1 isomer)
    • The cis isomer lacks a plane of symmetry and is chiral, existing as a pair of enantiomers (d- and l-forms). (2 isomers)
  • Total Isomers: 1 (trans) + 2 (cis-d, cis-l) = 3.

(D) [Cr(SCN)2(NH3)4]+ This is an [MA4B2] type complex, where SCN- is an ambidentate ligand. The coordination number is 6.

  • Geometrical Isomerism: It can exhibit cis and trans isomers for any given bonding mode of SCN-. (2 isomers: cis, trans)
  • Linkage Isomerism: The SCN- ligand can bind through sulfur (thiocyanato, -SCN) or through nitrogen (isothiocyanato, -NCS). We can have three types of linkage arrangements for the two SCN ligands:
    1. Both SCN ligands are S-bonded: [Cr(SCN)2(NH3)4]+ (thiocyanato)
      • cis-[Cr(SCN)2(NH3)4]+
      • trans-[Cr(SCN)2(NH3)4]+
    2. Both SCN ligands are N-bonded: [Cr(NCS)2(NH3)4]+ (isothiocyanato)
      • cis-[Cr(NCS)2(NH3)4]+
      • trans-[Cr(NCS)2(NH3)4]+
    3. One SCN is S-bonded and one is N-bonded: [Cr(SCN)(NCS)(NH3)4]+
      • cis-[Cr(SCN)(NCS)(NH3)4]+
      • trans-[Cr(SCN)(NCS)(NH3)4]+
  • Optical Isomerism: The cis and trans forms of [MA4B2] and [MA4BC] type complexes (like those formed here) are generally achiral due to the presence of planes of symmetry.
  • Total Isomers: 2 (cis/trans for S-bonded) + 2 (cis/trans for N-bonded) + 2 (cis/trans for mixed bonding) = 6.

Comparing the total number of isomers: (A) 2 (B) 1 (C) 3 (D) 6

The maximum number of isomers is 6, given by option (D).