A coordination compound consists of a central atom or ion , usually a metal known as the coordination center. It is surrounded by an array of bound molecules or ions known as ligands.Two or more compounds that have the same chemical formula but a different arrangement of atoms are known as isomers. Due to this difference in arrangement of atoms, coordination compounds pre-dominantly exhibit two types of isomerism namely, structural isomerism and stereo isomerism.
Stereoisomerism: Coordination compounds which have the same chemical or chemistry formula and chemical bonds but have different spatial arrangement are known as stereoisomers. These are further divided into geometrical and optical isomerism.
- Geometrical isomerism: Geometrical isomerism is observed in heteroleptic complexes (complexes with more than one type of ligands) due to different possible geometric arrangements of the ligands. This behavior is mainly observed in coordination compounds having coordination numbers equal to 4 and 6. For example: in a square planar coordination compound of formula [MX2L2] (where X and L are unidentate), the two ligands X may be arranged adjacent to each other in a “cisisomer”, or opposite to each other in a “trans isomer”. In octahedral complexes of the type [Ma3b3] (for example: [Co(NH3)3(NO2)3]), if three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial isomers. When they occupy the positions around the meridian of the octahedron, we get the meridional isomers.
- Optical isomerism: Two compounds with same chemical formula such that their mirror images are not superimposable on each other are known as optical isomers or enantiomers. Depending on the direction they rotate the plane of polarised light in polarimeter, the two forms of the compound are known as dextro (rotates to right) or laevo (rotates to left).
Structural isomerism:Structural isomerism is exhibited by the coordination compounds having same chemical formula but different arrangement of atoms. These are further divided into four types:
- Linkage isomerism: Linkage isomerism is exhibited by coordination compounds having ambidentate ligands (ligands which can bind to central atom from more than one ends). For example: NCS–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN.
- Coordination isomerism: Incoordination isomerism, the interchange of ligands between cationic and anionic entities of different metal ions present in coordination compounds takes place. For example: [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].
- Ionisation isomerism: Ionisation isomerism arises when the counter ion in a complex salt which is a potential ligand replacesthe ligand. For example: [Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4.
- Solvate isomerism: Solvate isomers are a special case of ionisation isomers in which compounds differdepending on the number of solvent molecule directly bonded to the metal ion. For example: [Cr(H2O)6]Cl3 and [Cr(H2O)5Cl]Cl2.H2O
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