The Foundations of Chemistry

Differences between structural isomersinvolve either more than one coordination sphere

or different donor atoms on the same ligand. They contain different atom-to-atom bonding

sequences.Simple stereoisomers of coordination compounds involve only one coordination

sphere and the same ligands and donor atoms. Before considering stereoisomers, we will

describe the four types of structural isomers.

STRUCTURAL (CONSTITUTIONAL) ISOMERS

Ionization (Ion–Ion Exchange) Isomers

These isomers result from the interchange of ions inside and outside the coordination

sphere. For example, red-violet [Co(NH 3 ) 5 Br]SO 4 and red [Co(NH 3 ) 5 SO 4 ]Br are ioniza-

tion isomers.

In structure A the SO 42 ion is free and is not bound to the cobalt(III) ion. A solution of

A reacts readily with a solution of barium, BaCl 2 , to precipitate BaSO 4 , but does not react

readily with AgNO 3. In structure B the SO 42 ion is bound to the cobalt(III) ion and so

it does not react with BaCl 2 in aqueous solution. The Brion is free, however, and a solu-

tion of B reacts with AgNO 3 to precipitate AgBr. Equimolarsolutions of A and B also have

different electrical conductivities. The sulfate solution, A, conducts electric current better

because its ions have 2
and 2charges rather than 1
and 1. Other examples of this

type of isomerism include

[Pt(NH 3 ) 4 Cl 2 ]Br 2 and [Pt(NH 3 ) 4 Br 2 ]Cl 2

[Pt(NH 3 ) 4 SO 4 ](OH) 2 and [Pt(NH 3 ) 4 (OH) 2 ]SO 4

[Co(NH 3 ) 5 NO 2 ]SO 4 and [Co(NH 3 ) 5 SO 4 ]NO 2

[Cr(NH 3 ) 5 SO 4 ]Br and [Cr(NH 3 ) 5 Br]SO 4

Hydrate Isomers

In some crystalline complexes, water can be insideand outsidethe coordination sphere. For

example, when treated with excess AgNO 3 (aq), solutions of the following three hydrate

isomers yield three, two, and one mole of AgCl precipitate, respectively, per mole of

complex.

Br

NH 3

H 3 N NH 3

H 3 N NH

3

2 

[Co(NH 3 ) 5 Br]SO 4

pentaamminebromocobalt(III)

sulfate

A

SO 42 

[Co(NH 3 ) 5 SO 4 ]Br

pentaamminesulfatocobalt(III)

bromide

B

Co Br

OSO 3

NH 3

H 3 N NH 3

H 3 N NH

3

Co

25-6

982 CHAPTER 25: Coordination Compounds

Isomers such as those shown here may
or may notexist in the same solution in
equilibrium. Such isomers are formed
by differentreactions.

We write the sulfato ligand as OSO 3
to emphasize that it is coordinated
through a lone pair on one of the O
atoms.

Recall that BaSO 4 (Ksp1.1 10 ^10 )
and AgBr (Ksp3.3 10 ^13 ) are
classified as insoluble in H 2 O, whereas
BaBr 2 is soluble and Ag 2 SO 4 (Ksp
1.7 10 ^5 ) is moderately soluble.

In A:

Ba^2 

SO 42 88nBaSO 4 (s)

Ag

SO 42 88nno rxn

In B:

Ag

Br88nAgBr(s)

Ba^2 

Br88nno rxn