1. CHM 122
Wk 12, II
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. Different “ways” to be isomers (very comparable to Tro, Fig. 24.6)
CHM 122
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Different “ways” to be isomers (very comparable to Tro, Fig. 24.6)
Structural
Optical Isomers or
Coordination Isomers
Geometric Isomers
 cis-trans
 same ligand, but connected via a different atom
 anionic ligand swaps with counter(an)ion
 will occur if species is “chiral” (no plane of symmetry)
 other “geometrical”
 for salts only
Omit in S’13

3. CHM 122
Wk 12, II
Example of Linkage Isomers (a type of Structural Isomerization) *The same ligand is attached, but via a different atom
Co(NH3)5(NO2)2+
Co(NH3)5(ONO)2+
donor atom

4. Coordination Isomers (the other kind of Structural type of isomer(s))
CHM 122
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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. Coordination Isomers (continued)
CHM 122
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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)
CHM 122
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Coordination Isomers (examples)
[Co(NH3)4Cl2]Br and [Co(NH3)4ClBr]Cl
[Co(NH3)4Cl2]Br  Co(NH3)4Cl Br -
[Co(NH3)4ClBr]Cl  Co(NH3)4ClBr Cl-
[Co(en)2Cl2]ox and [Co(en)2(ox)]Cl2

7. Coordination Isomers (different kind of example)
CHM 122
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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]
1st dissociates, get: Ru(en)33+ and Fe(CN)63-
2nd dissociates, get: Ru(en)2(CN)2+ and
Fe(en)(CN)4-

8. Two Families of Stereoisomer
CHM 122
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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. Example of (one kind of) Geometric Isomerism (cis-trans type)
CHM 122
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Example of (one kind of) Geometric Isomerism (cis-trans type)
All bonds same, but relative spatial arrangement differs (Cl’s 90° vs 180° degrees “apart“)

10. Cis-trans isomerization can occur in octahedral geometry also
CHM 122
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Cis-trans isomerization can occur in octahedral geometry also
cis
trans

11. CHM 122
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Orientation Needs to Be Considered! If structures are superimposable (i.e., identical), then not isomers
Both are cis (identical)
Both are trans (identical)

12. Not all complexes have a geometrical isomer!
CHM 122
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Not all complexes have a geometrical isomer!

13. Yet another kind of Geometric Isomer Pair (not cis-trans)*
CHM 122
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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. CHM 122
Wk 12, II
The following three slides are not needed for S’13, but I am including them for completeness

15. Enantiomers and “Chirality”
CHM 122
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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. Enantiomers and Chirality--Examples
CHM 122
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Enantiomers and Chirality--Examples
achiral => mirror image is identical, so no enantomer
chiral => has a mirror image that is not identical

17. CHM 122
Wk 12, II