8. Isomerism & Chirality

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10 Terms

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What are the two types of isomerism and their sub categories?

  • Structural isomerism - where the actual bonds are different - linkage isomerism, ionisation isomerism, solvate isomerism, coordination isomerism

  • Stereoisomerism - molecules with the same molecular and structural formula, but differ in how they are arranged in 3D - geometrical isomerism and optical isomerism

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What is linkage isomerism?

  • Structural isomerism

  • Ligands may be able to bond through a different donor atom (ambidentate ligands)

  • For example, NO2- can be bonded through the N (nitro) or one of the Os (nitrito)

  • The same with M-SCN, bonded through S is thiocyanato, or bonded through N is isothiocyanato

  • Similar with CN-, bonded through C is cyano, N is isocyano

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What is symbiosis?

  • In linkage isomerism with ambidentate ligands, the harder/softer donor atom will bond depending on the hardness/softness of the central metal ion

  • A hard ligand can also increase the hardness of the metal ion and attractive more hard ligands and vice versa with a soft ligand.

<ul><li><p>In linkage isomerism with ambidentate ligands, the harder/softer donor atom will bond depending on the hardness/softness of the central metal ion </p></li><li><p>A hard ligand can also increase the hardness of the metal ion and attractive more hard ligands and vice versa with a soft ligand. </p></li></ul><p></p>
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What is ionisation isomerism?

  • Structural isomerism

  • Compounds that contain the same atoms but will react differently depending on which ion is actually acting as a ligand and what is outside the square bracket as just a counter ion. - only things inside teh square bracket are covalently bonded

  • For example, [CoBr(NH3)5]SO4 or [CoSO4(NH3)5]Br

  • In one case, Br is the counter ion and this can produce a salt when reacted with silver, however is SO4 was outside the square bracket as a counter ion, it would not react with silver to form a salt.

<ul><li><p>Structural isomerism </p></li><li><p>Compounds that contain the same atoms but will react differently depending on which ion is actually acting as a ligand and what is outside the square bracket as just a counter ion. - only things inside teh square bracket are covalently bonded</p></li><li><p>For example, [CoBr(NH<sub>3</sub>)<sub>5</sub>]SO<sub>4 </sub>or [CoSO<sub>4</sub>(NH<sub>3</sub>)<sub>5</sub>]Br</p></li><li><p>In one case, Br is the counter ion and this can produce a salt when reacted with silver, however is SO4 was outside the square bracket as a counter ion, it would not react with silver to form a salt.</p></li></ul><p></p>
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What is Solvate isomerism?

  • Structural isomerism

  • A special case of ionisation isomerism where there are neutral solvate molecules - H2O

  • Can also be called hydrate isomerism

<ul><li><p>Structural isomerism </p></li><li><p>A special case of ionisation isomerism where there are neutral solvate molecules - H<sub>2</sub>O </p></li><li><p>Can also be called hydrate isomerism </p></li></ul><p></p>
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What is coordination isomerism?

  • Structural isomerism

  • Where ligands may interchange between cationic and anionic complexes

  • A joint double salt can have two complexes where the metals swap ligands and the overall empirical formula would be identical but the complexes within it are different.

<ul><li><p>Structural isomerism</p></li><li><p>Where ligands may interchange between cationic and anionic complexes</p></li><li><p>A joint double salt can have two complexes where the metals swap ligands and the overall empirical formula would be identical but the complexes within it are different. </p></li></ul><p></p>
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What is cis/trans geometrical isomerism? What geometries can this occur under?

  • Stereoisomerism

  • The occurrence of cis and trans complexes

  • Does NOT occur in tetrahedral complexes

  • Can be seen in square planar, square pyramidal and octahedral if it has MA2B4 formula

  • For octahedral, both of the A2 atoms will be axial to be trans - cis-trans isomerism within octahedral molecules can cause colour changes in the complex

<ul><li><p>Stereoisomerism </p></li><li><p>The occurrence of cis and trans complexes </p></li><li><p>Does NOT occur in tetrahedral complexes </p></li><li><p>Can be seen in square planar, square pyramidal and octahedral if it has MA<sub>2</sub>B<sub>4</sub> formula </p></li><li><p>For octahedral, both of the A<sub>2 </sub>atoms will be axial to be trans - cis-trans isomerism within octahedral molecules can cause colour changes in the complex </p></li></ul><p></p>
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What form of geometric isomerism can be seen in trigonal bipyramidal and square planar complexes?

  • axial/apical or equatorial/basal position isomerism

  • particular ligands may take the axial or equatorial positions and this can in some cases change the colour of the complex.

<ul><li><p>axial/apical or equatorial/basal position isomerism </p></li><li><p>particular ligands may take the axial or equatorial positions and this can in some cases change the colour of the complex. </p></li></ul><p></p>
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What layout of ligands can occur for a MA3B3 octahedral complex?

  • They can be facial or meridional

  • Facial is when the same three ligands take up the corners of a triangle to make a ‘face’ of the triangle

  • Meridional is when three ligands for a line from axial to equatorial to axial

<ul><li><p>They can be facial or meridional </p></li><li><p>Facial is when the same three ligands take up the corners of a triangle to make a ‘face’ of the triangle </p></li><li><p>Meridional is when three ligands for a line from axial to equatorial to axial </p></li></ul><p></p>
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What is optical isomerism? How can it arise in tetrahedral and octahedral complexes?

  • stereoisomerism

  • Molecules with the same molecular and structural formula but different arrangements

  • Tetrahedral complexes rarely show chirality due to high liability of tetrahedral complexes leading to fast racemisation - but it can be forced with ambidentate ligands as they are more stable and less liable

  • In octahedral complexes, there is lambda or delta forms of each optical isomer

<ul><li><p>stereoisomerism</p></li><li><p>Molecules with the same molecular and structural formula but different arrangements</p></li><li><p>Tetrahedral complexes rarely show chirality due to high liability of tetrahedral complexes leading to fast racemisation - but it can be forced with ambidentate ligands as they are more stable and less liable </p></li><li><p>In octahedral complexes, there is lambda or delta forms of each optical isomer</p></li></ul><p></p>