Compounds that have an equivalent formula but different structural arrangements are called isomers. Two or more different compounds having an equivalent formula are called isomers. Two principal sorts of isomerism are known among coordination compounds. Each of which may be further subdivided.

Stereoisomers

Stereoisomers have equivalent atoms, same sets of bonds, but differ within the relative orientation of those bonds. Ignoring special cases involving esoteric ligands, stereoisomers are divided into two parts:

• Geometrical isomerism
• Optical isomerism

Geometric isomers are possible for both square planar and octahedral complexes, but not tetrahedral. 

Optical isomers are possible for both tetrahedral and octahedral complexes, but not square planar.

Geometrical Isomerism in Coordination Compounds

The substituent groups could also be adjacent or opposite one another in disubstituted complexes. This results in geometric isomerism. As a result, square planar complexes like [Pt(NH3)4Cl2] are often prepared cis and trans. 

When the chlorine atoms are adjacent to every other it’s called cis form and It’s called trans form isomerism when two chlorine atoms are opposite each other. This type of isomerism occurs mostly in heteroleptic complexes due to the varied geometric arrangements of ligands around the central metal atom.

This type of isomerism is especially found in coordination compounds with coordination numbers 4 and 6.

• In a square planar complex (i.e. coordination compounds with coordination number 4 which have [MX2L2] type formula (X and L are unidentate ligands), the 2 ligands X could also be present adjacent to every other during a cis isomer, or opposite to every other to make a trans isomer.
• Square planar complex with MABXL type formula (where A, B, X, L are unidentate ligands) shows three isomers-two cis and one trans.
• Sis trans isomerism isn’t possible for a tetrahedral geometry.
• Cis-trans isomerism does exist in octahedral complexes. Two ligands X could be oriented cis or trans to each other in complexes having the formula [MX2L4].
• This type of isomerism is additionally observed when bidentate ligands L–L [e.g., NH2 CH2 CH2 NH2 (en)] are present in complexes with [MX2(L–L)2] type formula
• There is another sort of geometrical isomerism which occurs in octahedral coordination entities with the [MA3B3] type formula. An example is [Co(NH3)3(NO2)3].
• If three donor atoms of equivalent ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer.
• When the positions are around the meridian of the octahedron, we get the meridional isomer
• Facial Isomers: a group of three ligands (similar) arranged on an octahedron altogether cis – fashion.
• Meridional Isomers: a group of three similar ligands arranged on an octahedron with one pair trans

Optical Isomerism in Coordination Compounds

Optical isomers are related as non-superimposable mirror images and differ within the direction with which they rotate plane-polarised light. These isomers are mentioned as enantiomers or enantiomorphs of every other and their non-superimposable structures are described as being asymmetric. 

For structures that don’t possess a plane of symmetry, the mirror images aren’t superimposable. Referred to as chiral structures, such molecules rotate a beam of polarised light. 

• If the beam is rotated to the proper (when looking along the beam within the direction of propagation), the substance is claimed to be dextrorotatory (or simply Dextro) and indicated by (+). 
• The substances that rotate the plane of polarised light to the left are called levorotatory or Levo and are denoted by (-). 
• Racemic mixtures produce no net rotation of the polarised light when equal amounts of each form are mixed together. 

Dichlorobis(ethylenediamine)cobalt(II) exists in two geometrical isomers. For the trans isomer, there’s a plane of symmetry that bisects the cobalt ion and therefore the ethylenediamine ligands, leaving one Cl on either side of the plane. However, the cis isomer has no plane of symmetry. Thus, two optical isomers exist.

Conclusion

The existence of coordination compounds with an equivalent formula but different arrangements of the ligands was crucial within the development of coordination chemistry. Two or more compounds with an equivalent formula but different arrangements of the atoms are called isomers. 

Because isomers usually have different physical and chemical properties, it’s important to understand which isomer we are handling if quite one isomer is feasible. As we’ll see, coordination compounds exhibit equivalent sorts of isomers as organic compounds, also as several sorts of isomers that are unique. 

Isomers are compounds with an equivalent formula but different structural formulas and don’t necessarily share similar properties. There are many various classes of isomers, like stereoisomers, enantiomers, and geometrical isomers. There are two main sorts of isomerism: structural isomerism and stereoisomerism (spatial isomerism).