Transcript Ppt08(Wk12)TM IV-Isomerism_S15

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”
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Different “ways” to be isomers
(very comparable to Tro, Fig. 24.6)
Structural
Optical Isomers or
Coordination Isomers
 same ligand, but
connected via a
different atom
 anionic ligand
swaps with
counter(an)ion
Geometric Isomers
 cis-trans
 other “geometrical”
 will occur if species
is “chiral” (no plane
of symmetry)
 for salts only
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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
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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 
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Coordination Isomers (continued)
• You can check to see if you have
coordination isomers by considering the
dissociation in water.
– When compounds 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!!!!
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Coordination Isomers (examples)
• [Co(NH3)4Cl2]Br and [Co(NH3)4ClBr]Cl
– [Co(NH3)4Cl2]Br  Co(NH3)4Cl2+ + Br – [Co(NH3)4ClBr]Cl  Co(NH3)4ClBr+ + Cl-
• [Co(en)2Cl2]ox and [Co(en)2(ox)]Cl2
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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)47
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 (but
are not “superimposable”—i.e., not “identical”)
– Relative spatial position of atoms is same, except
“inverted” (mirror image)
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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“)
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Cis-trans isomerization can occur
in octahedral geometry also
cis
trans
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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)
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Not all complexes have a
geometrical isomer!
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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
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• The following three slides are not needed
for S’13, but I am including them for
completeness
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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
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Enantiomers and Chirality--Examples
chiral => has a mirror image that is not identical
achiral => mirror
image is identical, so
no enantomer
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