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Geometric Isomers

Geometric isomers are two or more coordination compounds which contain the same molecular formula, and bonds, but have different spatial ordering of the atoms. The square planar molecule, Pt(NH3)2Cl2, the two ammonia ligands (or the two chloride ligands) can be adjacent to one another or opposite one another. To change between the two isomers bonds must be broken and new bonds formed.

These two structures contain the same molecular struture and bonds but are non-superimposable. The isomer in which like ligands are adjacent to one another is called the cis isomer. The isomer in which like ligands are opposite one another is called the trans isomer.

= Pt

= N

= H

= Cl

cis-Pt(NH3)2Cl2

The two ammine ligands are adjacent to one another and the two chloro ligands are adjacent to one another. This compound is used medicinally to treat certain types of cancer. (tradename: "cisplatin").

trans-Pt(NH3)2Cl2

The two ammine ligands are opposite one another and the two chloro ligands are opposite one another. The like ligands are on opposite sides of the central metal ion. This compound does not exhibit any anti-tumor activity.

For the common structures which contain two or more different ligands, geometric isomers are possible only with square planar and octahedral structures. Geometric isomers do not exist for linear and tetrahedral structures.

= Co

= N

= H

= Cl

cis-[Co(NH3)4Cl2]+

In the cis isomer the two chloro ligands are adjacent to one another in this octahedral complex ion. To move form one like ligand to the other it

trans-[Co(NH3)4Cl2]+

In the trans isomer the two chloro ligands are opposite one another in this complex ion. To move from one like ligand to the other the plane

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