Geometric Isomers in Coordination Chemistry

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Stereoisomers

Stereoisomers are molecules that have the same molecular formula and the same connectivity of atoms, but differ only in the three-dimensional arrangement of those atoms in space. Two general forms of stereoisomerism are optical isomers or enantiomers, and geometric isomers.

Geometric Isomers

Geometric isomers differ in the relative orientation of substituents. In coordination chemistry, this usually means differing arrangement of ligands around the coordination sphere central metal. To describe sets of geometric isomers, it is usually sufficient to determine the relative position of only two or three of these ligands.

The prefixes cis- and trans- can be used to describe the relative position of two ligands. This is most common in square planar and octahedral geometries, where two groups will either be 90° (cis-) or 180° (trans-) apart.

 
cis-diamminedichloroplatinum(II)
cis-[PtCl2(NH3)2]
  trans-diamminedichloroplatinum(II)
trans-[PtCl2(NH3)2]

 
cis-tetraamminedichlorochromium(III)
cis-[CrCl2(NH3)4]+
  trans-tetraamminedichlorochromium(III)
trans-[CrCl2(NH3)4]+

The prefixes fac- and mer- can be used to describe the relative position of three ligands in an octahedral complex. The three groups can either occupy a triangular face of the octahedron (a facial arrangement), or they can sit in three coplanar positions, forming an arc across the coordination sphere that can be thought of as a meridion (the meridional arrangement).

 
fac-trichlorotripyridinerhodium(III)
fac-[RhCl3(py)3]
  mer-trichlorotripyridinerhodium(III)
mer-[RhCl3(py)3]


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