The Foundations of Chemistry

ClXPtXCl angle
90° ClXPtXCl angle
180°

NXPtXN angle
90° NXPtXN angle
180°

Cl NH 3

H 3 N Cl

trans

dark yellow

Pt

Cl NH 3

Cl NH

3

cis

pale yellow

Pt

cis-diamminedichloroplatinum(II),
cis-[Pt(NH 3 ) 2 Cl 2 ]

25-7 Stereoisomers 985

In the cisisomer, the chloro groups are closer to each other (on the same side of the
square) than they are in the transisomer. The ammine groups are also closer together in
the ciscomplex.
Geometric isomerism is possible for octahedral complexes. For example, complexes of
the type MA 4 B 2 can exist in two isomeric forms. Consider as an example the complex ion
[Co(NH 3 ) 4 Cl 2 ]
. The two like ligands (Cl) can be either cisor transto each other. These
two complexes are different colors: solutions and salts of the cisisomer are violet and those
of the transisomer are green.

NH 3

Cl

Cl NH

3

NH 3

NH 3

cis-tetraamminedichlorocobalt(III) ion,

cis-[Co(NH 3 ) 4 Cl 2 ]

Co

A stereoview of cis-tetraamminedichlorocobalt(III) ion, cis-[Co(NH 3 ) 4 Cl 2 ]
. The two chloro

ligands are adjacent to each other.

trans-diamminedichloroplatinum(II),

trans-[Pt(NH 3 ) 2 Cl 2 ]

The cisisomer has been used

successfully in cancer chemotherapy.

The transisomer has no such activity.

Cis–transisomerism is not possible for

tetrahedral complexes, in which all

angles are (ideally) 109.5°.