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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)

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 IsomersCoordination IsomersOptical Isomers or

Omit in S’13

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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 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!!!!

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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)6

3-

– 2nd dissociates, get: Ru(en)2(CN)2+ and

Fe(en)(CN)4-

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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)

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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 identicalachiral => mirror image is identical, so no enantomer

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