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Geometric and Optical Isomerism

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Transition metals can generate a wide range of complex compounds by sharing electrons with a variety of anions or neutral molecules, such compounds are referred to as coordination compounds in present terminology.

New ideas on chemical bonds and molecular structure have shed light on how these substances act as essential parts of biological systems. The coordination compounds of magnesium, iron, and cobalt are chlorophyll, haemoglobin, and vitamin B12, respectively. Coordination compounds are used as analytical reagents, industrial catalysts, and various metallurgical processes. Additionally, there are numerous uses for coordination compounds in the electroplating, textile dyeing, and pharmaceutical sector.

What is Isomerism?

When two or more chemical compounds share the same molecular formula, this phenomenon is known as isomorphism. Greek terms isos, which means “equal,” and meros, which means “part,” are the roots of the word “isomerism.” When two or more compounds have the same molecular formula but different chemical and physical properties, this is referred to as isomorphism. One of the causes of the existence of many organic molecules is isomerism, which is a common phenomenon in organic chemistry. It describes substances with the same molecular formula but differing physical or chemical properties

Isomerism in Coordination compounds

Two or more compounds with the same chemical formula but a distinct atom arrangement are said to be isomers. They have one or more different physical or chemical properties as a result of the varied atom arrangements. Due to the many bond types and challenging formulae of many coordination compounds, we find a variety of isomers.

If more than one isomer is available, it is essential to know which one we are working with because isomers typically have different physical and chemical properties. We shall observe that coordination compounds display many of the same isomers as organic compounds and some unique isomers.

 Among coordination compounds, there are two main kinds of isomers that are identified.

  • Stereoisomerism
    • Geometrical Isomerism 
    • Optical Isomerism
  • Structural isomerism
    • Linkage Isomerism
    • Coordination Isomerism
    • Ionization Isomerism
    • Solvate Isomerism

Stereoisomerism

Stereoisomers share the same chemical structure and chemical interactions, but they are arranged differently in space. Isomer refers to two or more compounds with the same formula but different atom arrangements. These isomers’ properties differ from compound to compound. Isomerism is the phenomenon that results in a variety of isomers. One kind of stereoisomerism is optical isomerism, and the second is geometric isomers.

Geometric Isomerism

The molecules’ spatial arrangement changes around the carbon atoms as a result of compounds having impeded rotation in space around a particular carbon-carbon double bond or a single bond. This type of isomerism is known as geometric isomerism, and the isomers are known as geometric isomers. 

Geometrical isomerism is also a type of stereoisomerism in which the identical substituent is bonded differently to the carbon-carbon double bond. They have different physical characteristics.

Geometric Isomerism (in complex with 4 coordinates bond)

Tetrahedral or square planar geometry are two possible configurations for complexes with a core atom and a 4-coordination number. Tetrahedral complexes cannot show geometric isomerism since all four ligands are at neighboring (cis) positions to one another and all four bond angles are the same (109.5o).

Geometrical Isomerism in Square Planar Complexes

Trans-isomer is a square planar complex with two similar ligands at opposite locations (180o apart), whereas cis-isomer is a square planar complex with similar ligands at adjacent positions (90o apart).

Geometrical Isomerism cis-form and trans-form

 

Geometrical Isomerism (in complex with 6-Coordinate Bond) or Octahedral Complexes

We know that the shape of a complex compound with a core atom and a coordination number of 6 is octahedral. Complexes of type [MA4B2], where A and B are monodentate ligands, can exist in both cis and trans isomeric forms.
Ions are in opposite positions in trans isomers, while in cis isomers two ions occupy the adjacent sites of the octahedron.

Geometrical Isomerism in Octahedral complexes

 

Despite having the same chemical formula, these cis and trans isomers have different physical characteristics. Because of their symmetry, trans isomers have higher melting points than cis isomers. Likewise, cis isomers have higher boiling points than trans isomers because they contain polarity brought on by a dipole moment.

Optical Isomerism or (d-form, l-form)

Stereoisomerism known as optical isomerism refers to compounds with the same chemical structure and connectivity but a distinct spatial arrangement. They have the same physical characteristics. They do not display cis and trans isomers in optical isomerism. In nature, optical isomers are optically active. A plane polarized light’s plane rotates through an angle, which can be to the left or to the right, when a solution of certain complicated substances is in its path. A compound is considered to be optically active if it has the ability to rotate the plane of polarized light, which is referred to as its optical activity.

  • When the plane of polarized light is rotated towards the right (in a clockwise direction) by the isomer, it is said to be dextro-rotatory or d-form.
  • Likewise, when the plane of polarized light is rotated towards the left (in an anti-clockwise direction) by the isomer, it is said to be levorotatory or l-form.
  • Optically inactive isomers are those that cannot rotate the plane of polarized light. Such an isomer is known as meso, racemic, or dl-form

The d- and l- forms are physically and chemically similar, yet they behave differently when exposed to polarized light.

Similar to how the left and right hands are mirror images of one another, the d and l shapes are the same.

The molecule must be asymmetric (have no plane of symmetry) and not be superimposable on its mirror image in order to exhibit optical isomerism. These two are the most important and essential requirements for this phenomenon.

Optical Isomer

 

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FAQs on Geometric and Optical Isomerism

Question 1: What are isomers?

Answer:

Isomers are compounds with the same chemical composition but different structural configurations.

Question 2: What are Cis-isomers and Trans-isomers?

Answer:

Cis Isomers: Similar groups are referred to as cis isomers when they are found on the same side of the double bonds.

Trans Isomers: Trans isomers are defined as having comparable groups on the opposing sides of double bonds.

Question 3: What are the different types of isomerism for coordination compounds?

Answer:

Different types of isomerism for coordination compounds are,

  • Geometric isomerism
  • Optical isomerism
  • Linkage isomerism
  • Coordination isomerism
  • Ionization isomerism
  • Solvate isomerism

Question 4: How does Geometric Isomerism occur in cyclic alkanes?

Answer:

Geometric isomerism in cyclic compounds requires at least two additional groups besides hydrogens on the ring, and these additional groups must be on distinct ring carbon atoms.

Question 5: Which compound shows Geometrical Isomerism?

Answer:

If a compound has two distinct groups on the same carbon atom that are doubly linked to another carbon atom or connected to a ring carbon atom, it exhibits geometric isomerism.

Question 6: Which compound shows Optical Isomerism?

Answer:

When two substances with the same molecular and structural formula cannot be superimposed on one another, optical isomerism primarily occurs.



Last Updated : 14 Feb, 2023
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