JEE Challenger
More from Coordination Compounds
ChemMedAdv 2016 • Paper 1

Geometric Isomers Count for Cobalt Unsymmetrical Bidentate Complex

Find the total number of geometric isomers possible for the coordination complex [CoL2Cl2]−[\text{CoL}_2\text{Cl}_2]^-, where L=H2NCH2CH2O−\text{L} = \text{H}_2\text{NCH}_2\text{CH}_2\text{O}^-.

Official Numerical Answer5

Topics & Concepts

Coordination CompoundsCoordination CompoundsIsomerism

Step-by-Step Solution

To find the total number of geometric isomers possible for the complex [CoL2Cl2]−[\text{CoL}_2\text{Cl}_2]^-, where L=H2NCH2CH2O−\text{L} = \text{H}_2\text{NCH}_2\text{CH}_2\text{O}^-, we can analyze the coordination environment of the octahedral complex.

1. Complex Structure and Ligand Classification

  • Central metal ion: Co3+\text{Co}^{3+} (coordination number = 6)
  • Monodentate ligand: C=Cl−\text{C} = \text{Cl}^-
  • Unsymmetrical bidentate ligand: AB=H2NCH2CH2O−\text{AB} = \text{H}_2\text{NCH}_2\text{CH}_2\text{O}^- with donor atoms:
    • A=N\text{A} = \text{N} (amino group)
    • B=O\text{B} = \text{O} (alkoxide group)

Thus, the complex can be represented in the general form [M(AB)2C2][M(\text{AB})_2\text{C}_2].

2. Constraint for Bidentate Ligands

For a chelating bidentate ligand AB\text{AB}, the donor atoms A\text{A} and B\text{B} belonging to the same ligand molecule must occupy cis positions in the octahedral geometry (i.e., they cannot span trans positions across the central metal).

3. Enumeration of Geometrical Isomers

An octahedral complex's geometrical isomerism is completely specified by the set of three pairs of ligands that are trans to each other:

Case I: The two Cl−\text{Cl}^- ligands are trans to each other

  1. (Cl,Cl)(N,N)(O,O)(\text{Cl}, \text{Cl})(\text{N}, \text{N})(\text{O}, \text{O})

    • Cl\text{Cl} is trans to Cl\text{Cl}
    • N\text{N} is trans to N\text{N}
    • O\text{O} is trans to O\text{O}

  2. (Cl,Cl)(N,O)(N,O)(\text{Cl}, \text{Cl})(\text{N}, \text{O})(\text{N}, \text{O})

    • Cl\text{Cl} is trans to Cl\text{Cl}
    • Each N\text{N} is trans to an O\text{O} atom of the other ligand

Case II: The two Cl−\text{Cl}^- ligands are cis to each other

  1. (Cl,N)(Cl,N)(O,O)(\text{Cl}, \text{N})(\text{Cl}, \text{N})(\text{O}, \text{O})

    • Both Cl\text{Cl} ligands are trans to N\text{N} atoms
    • O\text{O} is trans to O\text{O}

  2. (Cl,O)(Cl,O)(N,N)(\text{Cl}, \text{O})(\text{Cl}, \text{O})(\text{N}, \text{N})

    • Both Cl\text{Cl} ligands are trans to O\text{O} atoms
    • N\text{N} is trans to N\text{N}

  3. (Cl,N)(Cl,O)(N,O)(\text{Cl}, \text{N})(\text{Cl}, \text{O})(\text{N}, \text{O})

    • One Cl\text{Cl} is trans to N\text{N}, the other Cl\text{Cl} is trans to O\text{O}
    • One N\text{N} is trans to an O\text{O}

Conclusion

Since all 5 sets of trans-pairs are distinct and physically realizable without straining the bidentate chelate rings, the total number of geometric isomers possible for [CoL2Cl2]−[\text{CoL}_2\text{Cl}_2]^- is 5.

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