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Introduction

Isomers:

are two or more molecules or ions that contain the same number and kinds of atoms, but the atoms are arranged differently (i.e., the structures are non-superimposable).
have the same molecular formula.
have different physical and/or chemical properties.
do not exist for all coordination compounds.

Structural isomers: contain the same number and kinds of atoms, but one or more bonds is/are different (i.e., the connectivity between atoms is different). There are two types of structural isomers:

coordination isomers, and
linkage isomers.

Stereoisomers: contain the same number and kinds of atoms, and the same number and kinds of bonds (i.e., the connectivity between atoms is the same), but the atoms are arranged differently in space. There are two types of stereoisomers:

geometric isomers, and
optical isomers.

Note that coordination isomers and linkage isomers can exist only with coordination compounds. Geometric isomers and optical isomers occur not only with coordination compounds but also with many organic compounds

Linkage Isomers

Linkage isomers are two or more coordination compounds in which the donor atom of at least one of the ligands is different (i.e., the connectivity between atoms is different).

This type of isomerism can only exist when the compound contains a ligand that can bond to the metal atom in two (or more) different ways. Some ligands that can form linkage isomers are shown below.

Not all coordination compounds have linkage isomers.

Linkage isomers have different physical and chemical properties.

Ligands That Can Form Linkage Isomers
ligand	Lewis structure	name	donor atoms
CN^-		cyanide ion	C or N
SCN^-		thiocyanate ion	S or N
NO₂^-		nitrite ion	N or O


[Co(NH₃)₄(NO₂)Cl]⁺ Note that the N atom of the nitrite group is bonded to the Co atom. The nitrite group is written as "NO₂" in the molecular formula (rather than "ONO") with the N atom nearest to the Co symbol to indicate that the N atom (rather than an O atom) is the donor atom. Note also the octahedral structure.	[Co(NH₃)₄(ONO)Cl]⁺ Note that one of the O atoms of the nitrite group is bonded to the Co atom. The nitrite group is written as "ONO" in the molecular formula (rather than "NO₂") with the O atom nearest to the Co symbol to indicate that the O atom is the donor atom. Note also the octahedral structure

Geometric Isomers

Geometric isomers are two or more coordination compounds which contain the same number and types of atoms, and bonds (i.e., the connectivity between atoms is the same), but which have different spatial arrangements of the atoms.

Not all coordination compounds have geometric isomers.

For example, in 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.

Note that these two structures contain the same number and kinds of atoms 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.


cis-Pt(NH₃)₂Cl₂ Note that the two NH₃ ligands and the two Cl ligands are adjacent to one another. This compound is used medicinally to treat certain types of cancer. (tradename: "cisplatin").	trans-Pt(NH₃)₂Cl₂ Note that the two NH₃ ligands and the two Cl ligands are opposite one another. 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 (i.e., geometric isomers cannot exist for linear and tetrahedral structures).


cis-[Co(NH₃)₄Cl₂]⁺ Note that the two chloride ligands are adjacent to one another in this octahedral complex ion. In aqueous solution, this complex ion has a violet color.	trans-[Co(NH₃)₄Cl₂]⁺ Note that the two chloride ligands are opposite one another in this complex ion. In aqueous solution, this complex ion has a green color.

Optical Isomers

Optical isomers are two compounds which contain the same number and kinds of atoms, and bonds (i.e., the connectivity between atoms is the same), and different spatial arrangements of the atoms, but which have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer. Molecules or ions that exist as optical isomers are called chiral.

Not all coordination compounds have optical isomers.

Pure samples of enantiomers have identical physical properties (e.g., boiling point, density, freezing point). Chiral molecules and ions have different chemical properties only when they are in chiral environments.

Optical isomers get their name because the plane of plane-polarized light that is passed through a sample of a pure enantiomer is rotated. The plane is rotated in the opposite direction but with the same magnitude when plane-polarized light is passed through a pure sample containing the other enantiomer of a pair.

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