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 ionC or N
SCN-thiocyanate ionS or N
NO2-nitrite ionN or O

= Co

= N

= H

= O

= Cl

[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.