Isomerism
 isomers: two or more molecules/ions that contain the same number and kinds of atoms, but
arranged differently.
o Isomers have the same molecular formula.
o Isomers have different chemical and/or physical properties.
o Isomers do not exist for every coordination compound.
I. Structural (Constitutional) Isomers
o
Contain same atoms, but bonds differ
A. Coordination Isomers
o composition of complex ion differs
e.g., [Cr(NH3)5(OSO3)]Br
vs.
[Cr(NH3)5Br]SO4
B. Linkage Isomers
o point of attachment of at least one ligand differs (i.e., different donor atoms)
e.g., [Co(NH3)4(NO2)Cl]Cl
vs.
[Co(NH3)4(ONO)Cl]Cl
ligands that can cause linkage isomerism:
SCN–
CN–
NO2–
OCN–
The above types of isomerism are applicable to coordination compounds only. The following two
types are applicable to organic compounds as well as coordination compounds.
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II. Stereoisomers
o Contain same atoms & types of bonds, but spatial arrangement of atoms differs
A. Geometric Isomers (“cis-trans isomers”, “diastereomers”)
e.g., Pt(NH3)2Cl2
In organic chemistry, cis-trans isomerism is also observed in many compounds with a
C=C bond.
B. Optical Isomers (“enantiomers”)
 non-superimposable mirror images
e.g., [ZnFClBrI]2–
Most common, especially in organic compounds—tetrahedral central atom with 4
different groups attached.
 enantiomers have identical physical properties
 enantiomers have different chemical properties ONLY in environments containing
other optical isomers.
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Optical isomerism also occurs in octahedral complexes containing multiple bidentate ligands.
Application: Platinum Anticancer Drugs
cisplatin: produced serendipitously in a study of effects of electric fields on mitotic cell division using
“inert” Pt electrodes and NH4Cl in gel.
cis-Pt(NH3)2Cl2
cisplatin family: neutral Pt(II) or Pt(IV) cpds. with two cis NH3 ligands
aquated derivative binds to adjacent guanine residues on DNA, blocking transcription complex and
triggering cell killing.
very successful in treatment of testicular cancers (e.g., Scott Hamilton, Lance Armstrong).
See Journal of Chemical Education article on class web site for much more technical detail.
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