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1 Isomers It is possible (actually fairly common, especially in organic chemistry) for two compounds (or complexes) to have the same formula, yet NOT be.

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Presentation on theme: "1 Isomers It is possible (actually fairly common, especially in organic chemistry) for two compounds (or complexes) to have the same formula, yet NOT be."— Presentation transcript:

1 1 Isomers It is possible (actually fairly common, especially in organic chemistry) for two compounds (or complexes) to have the same formula, yet NOT be the same chemical substance (or species). –How can you tell? At least one property is different! (You can tell operationally) –How can this be? ANS: Something is different about the way the atoms in the species are arranged. There can be MANY ways for this to happen! Different Connections between atoms (different bonds) Same connections, but different arrangement in space Two compounds (species) are called isomers if they have the same formula but are not the same

2 2 Different ways to be isomers (very comparable to Tro, Fig. 24.6) cis-trans other geometrical will occur if species is chiral (no plane of symmetry) anionic ligand swaps with counter(an)ion for salts only same ligand, but connected via a different atom Structural Geometric Isomers Coordination Isomers Optical Isomers or Omit in S13

3 3 Example of Linkage Isomers (a type of Structural Isomerization) * The same ligand is attached, but via a different atom Co(NH 3 ) 5 (NO 2 ) 2+ Co(NH 3 ) 5 (ONO) 2+ donor atom

4 4 Coordination Isomers (the other kind of Structural type of isomer(s)) The word coordination implies that somebody different is coordinating (not just a different atom, but a different ligand). [still a different connection or bond] –How can there be a different ligand if the formula must be the same? ANS: You move a ligand out and put a counterion* in. I called this swapping Unlike linkage isomers (in which no counterions need be shown), one can only have coordination isomers with coordination compounds that are salts (i.e., counterions must be present/shown) * You can also swap ligands in the special case in which both the cation and anion are metal complex ions. See later

5 5 Coordination Isomers (continued) You can check to see if you have coordination isomers by considering the dissociation in water. –When complex is dissolved in water, the complexes will have a different formula (only the compounds are isomers here) Remember, you cannot swap a neutral ligand with a counter anion!!!!

6 Coordination Isomers (examples) [Co(NH 3 ) 4 Cl 2 ]Br and [Co(NH 3 ) 4 ClBr]Cl –[Co(NH 3 ) 4 Cl 2 ]Br Co(NH 3 ) 4 Cl Br - –[Co(NH 3 ) 4 ClBr]Cl Co(NH 3 ) 4 ClBr + + Cl - [Co(en) 2 Cl 2 ]ox and [Co(en) 2 (ox)]Cl 2 6

7 Coordination Isomers (different kind of example) *As noted earlier, in the special case in which both the cation and anion are metal complexes, one can get coordination isomers by simply swapping ligands. –The key is that the ligands coordinating to each metal are now different. Different connection [Ru(en) 3 ][Fe(CN) 6 ] & [Ru(en) 2 (CN) 2 ][Fe(en)(CN) 4 ] –1 st dissociates, get: Ru(en) 3 3+ and Fe(CN) 6 3- –2 nd dissociates, get: Ru(en) 2 (CN) 2 + and Fe(en)(CN) 4 - 7

8 8 Two Families of Stereoisomer In both, all bonds are the same, but something is different about the relative positions of the atoms in space Geometric: not mirror images –Relative spatial position of atoms is different –Cis-trans, or other geometric isomer Optical (Enantiomers) : ARE mirror images –Relative spatial position of atoms is same, except inverted (mirror image)

9 9 Example of (one kind of) Geometric Isomerism (cis-trans type) All bonds same, but relative spatial arrangement differs (Cls 90° vs 180° degrees apart)

10 10 Cis-trans isomerization can occur in octahedral geometry also cis trans

11 11 Orientation Needs to Be Considered! If structures are superimposable (i.e., identical), then not isomers Both are cis (identical) Both are trans (identical)

12 12 Not all complexes have a geometrical isomer!

13 13 Yet another kind of Geometric Isomer Pair (not cis-trans)* Same # and type of each bond, but different relative spatial arrangement of atoms *Tro (correctly) calls these fac and mer. You do not need to know these names, but just be able to recognize that these are geometric isomers

14 The following three slides are not needed for S13, but I am including them for completeness 14

15 15 Enantiomers and Chirality A molecule or complex is chiral if –There is NO plane of symmetry –Its mirror image is NOT the same structure (i.e., NOT superimposable A molecule or complex is achiral if –There IS one (or more) plane of symmetry –Its mirror image IS the same exact structure (i.e., superimposable) Enantiomers are isomers that are mirror images, THUS: –If a structure IS chiral, it WILL have an enantiomer –If a structure is achiral, it will NOT have an enantiomer

16 16 Enantiomers and Chirality--Examples chiral => has a mirror image that is not identical achiral => mirror image is identical, so no enantomer

17 17


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