Isomerism in Coordination Compounds:
The two or more compounds having the same chemical formula but different properties (physical or chemical or both) are known as “Isomers” and the phenomenon is termed as Isomerism. Isomers can be broadly classified into two major classes-
(1) Structural Isomers.
(2) Stereo Isomers.
“These are those isomers which have different structural arrangements around the central metal atom”. The various types of structural isomers are as follows-
(a) Ionisation Isomers- “Coordination Compounds which have the same molecular formula but yield different ions in solution are known as Ionisation Isomers” i.e. there is an interchange of groups between the coordination sphere of the metal ion and the ions present outside this coordination sphere. Example- violet pentammine bromo cobalt (III) sulphato, [Co(NH3)5Br]+2SO4-2 is an ionisation isomer of red pentammine sulphato cobalt (III) bromide, [Co(NH3)5SO4]+Br–. The violet compound gives [Co(NH3)5Br]+2 and SO4-2 ions in an aqueous solution and so it gives a white precipitate with BaCl2 (test for SO4-2 ions) whereas red compound gives [Co(NH3)5SO4]+ and Br– ions in aqueous solution and gives a pale yellow precipitate with AgNO3 (test for Br– ions). Other examples are-
- [Pt(NH3)4Cl2]Br2 and [Pt(NH3)4Br2]Cl2
- [Co(NH3)5NO3]SO4 and [Co(NH3)5SO4]NO3
(b) Coordination Isomers- This type of isomerism arises due to the interchange of ligands within the coordination sphere itself. It is shown by compounds in which both cation and anion are complexes. Example- [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] are coordination isomers. Other examples are-
- [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4]
- [Pt(NH3)4][PtCl4] and [Pt(NH3)3Cl][PtCl3(NH3)]
(c) Linkage Isomers- This type of isomerism is shown by complexes containing monodentate ligands capable of coordinating in more than one way (also known as Ambidentate Ligands) and so they differ only in their point of attachment with the central metal atom. Example- complexes containing NO2– (coordinate through N- known as nitro group or O- known as nitrito group) and SCN– (coordinate through S or N) ions can show this type of isomerism. Example-
- Pentammine nitro cobalt (III) chloride, [Co(NH3)5NO2]Cl2 (yellow brown) and Pentammine nitrito cobalt (III) chloride, [Co(NH3)5ONO]Cl2 (red) are linkage isomers.
(d) Hydrate Isomers- This type of isomerism arises because of the different positions taken up by the water molecules inside and outside the coordination sphere giving a number of hydrate isomers. Example- [Cr(H2O)6]Cl3 (violet);[Cr(H2O)5Cl]Cl2.H2O, Pentaquo chloro chromium (III) chloride hydrate (blue-green); and [Cr(H2O)4Cl2]Cl.2H2O, Dihydrate (green) are hydrate isomers. Other examples are-
- [Co(NH3)3(H2O)2Cl]Br2 and [Co(NH3)3(H2O)ClBr]Br.H2O
- [Co(en)2(H2O)Cl]Cl2 and [Co(en)2Cl2]Cl.H2O
“These isomers contain same atoms and the same atom to atom bonding but differ only in the spatial arrangement of atoms about the central metal atom”. This type of isomerism is exhibited by complexes with a coordination number of four or more. It is mainly of two types-
(a) Geometrical Isomerism- This type of isomerism occurs in those coordination compounds which have heterogeneous ligands. Example- [Pt(NH3)2Cl2] and [Pt(NH3)2Cl4] because in these compounds ligands occupy different positions around the central ion. The isomer in which the same kind of ligands is adjacent to each other is known as Cis-isomer while it is known as Trans-isomer if ligands of the same kind are opposite to each other. The tetrahedral complexes with coordination numbers equal to 4 do not show Geometrical Isomerism because the relative positions of the atoms w.r.t. each other will be the same. In square planar complexes (coordination number = 4), cis-trans isomerism is exhibited. Example in [Pt(NH3)2(Cl)2].
In octahedral complexes (coordination number = 6), cis and trans isomers are possible. Example in [Co(NH3)4Cl2]+ two isomers are possible.
(b) Optical Isomerism- This type of isomerism arises due to the tendency of certain compounds to rotate the plane-polarized light (having vibrations only in one direction) compounds having the same molecular and structural formula but differ in their behaviour towards the plane-polarized light are called optical isomers. The form which rotates the plane of polarized light towards the right-hand side is called dextrorotatory [d or (+)] form and the form which rotates the plane towards the left-hand side is called laevorotatory [l or (-)] form. The phenomenon is known as optical isomerism. Optical isomers have identical physical and chemical properties. The -d and -l forms are mirror images of each other just as the left-hand side is the mirror image of the right-hand side. Example-