The Foundations of Chemistry

STEREOISOMERS

Compounds that contain the same atoms and the same atom-to-atom bonding sequences,

but that differ only in the spatial arrangements of the atoms relative to the central atom,

are stereoisomers.Complexes with only simpleligands can exist as stereoisomers only if

they have coordination number 4 or greater. The most common coordination numbers

among coordination complexes are 4 and 6, and so they will be used to illustrate stereoiso-

merism.

Geometric (cis–trans) Isomers

In geometric isomers,or cis-transisomers,of coordination compounds,the same ligands

are arranged in different orders within the coordination sphere. Geometric isomerism

occurs when atoms or groups of atoms can be arranged on two sides of a rigid structure.

Cismeans “adjacent to” and transmeans “on the opposite side of.” Cis- and trans-

diamminedichloroplatinum(II) are shown below.

25-7

NH 3

H 3 N

NO

O

H 3 N

H 3 N NH

3

2 
 2 

[Co(NH 3 ) 5 NO 2 ]Cl 2

pentaamminenitrocobalt(III)

chloride

yellow, stable in acids

2 Cl

[Co(NH 3 ) 5 ONO]Cl 2

pentaamminenitritocobalt(III)

chloride

red, decomposes in acids

2 Cl

NH 3

H 3 N

Co

H 3 N

H 3 N NH

Co^3

O

N O

Pentaamminenitrocobalt(III) ion,

[Co(NH 3 ) 5 NO 2 ]^2 

984 CHAPTER 25: Coordination Compounds

Pentaamminenitritocobalt(III) ion,

[Co(NH 3 ) 5 ONO]^2 

A complex with coordination number
2 or 3 that contains only simple ligands
can have only one spatial arrangement.
Try building models to see this.