22.5: Structure and Isomerism in Coordination Compounds

Structure and Isomerism in Coordination Compounds:

Constitutional Isomers: isomers that differ in how the atoms are joined together, specifically, in the order that they are bonded to each other.

Stereoisomers: isomers in which the atoms are bonded to each other in the same order but differ in the precise arrangement of the atoms in space.


Constitutional Isomers:

Example: Ion-Exchange Isomer

[Co(NH₃)₅(SO₄)]Br
[Co(NH₃)₅Br]SO₄

In the first isomer, SO₄ is attached to the Cobalt and is part of the complex ion (the cation), with Br as the anion. In the second isomer, Br is attached to the cobalt as part of the complex and SO₄ is acting as the anion.

Example: Coordination Isomer

[Cu(NH₃)₄][PtCl₄]
[Pt(NH₃)₄][CuCl₄]

Here, both the cation and anion are complex ions. In the first isomer, NH₃ is attached to the copper and the Cl^– are attached to the platinum. In the second isomer, they have swapped.

Example: Linkage Isomer

[FeCl₅(NO₂)]^3–
[FeCl₅(ONO)]^3–

Here, the difference is in how the ligand bonds to the metal. In the first isomer, the ligand bonds to the metal through an electron pair on the nitrogen. In the second isomer, the ligand bonds to the metal through an electron pair on one of the oxygen atoms. It's easier to see it:




Stereoisomerism:

Geometric Isomers: isomers in which the atoms are joined to one another in the same way but differ because some atoms occupy different relative positions in space.



In the cis arrangement, two like ligands are placed next to each other. In the trans arrangement, two like ligands are placed across from each other.




(helpful source)

(info on mer/fac isomers)


Chiral vs. Achiral:



Achiral Molecules: these isomers are superimposable mirror images of one another. A molecule and it's mirror image can completely overlap.

Chiral Molecules: If a molecule and a molecule that appears as its mirror image cannot completely overlap, they are chiral molecules. These isomers are non-superimposable mirror images of one another. Also called optical isomers, optically active, or enantiomers.

Enantiomers have identical properties, but are differentiated by their effect on plane-polarized light.



Dextrorotatory: a compound whose solution rotates the plane of polarized light to the right (when looking toward the source of light).

Levorotatory: a compound whose solution rotates the plane of polarized light to the left (when looking toward the source of light).

Racemic Mixture: a mixture of equal amounts of optical isomers. Because the two isomers rotate the plane of polarized light by the same angle in opposite directions, they cancel each other out and have no net effect.