The Foundations of Chemistry

trans-tetraamminedichlorocobalt(III) ion,

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

Co

NH 3

NH 3

Cl NH 3

H 3 N Cl

A stereoview of trans-tetraamminedichlorocobalt(III) ion, trans-[Co(NH 3 ) 4 Cl 2 ]
. The
two chloro ligands are across from each other.

986 CHAPTER 25: Coordination Compounds

Figure 25-2 The dichlorobis(ethylenediamine)cobalt(III) ion, [Co(en) 2 Cl 2 ]
,

exists as a pair of cis–transisomers. Ethylenediamine is represented as.

Cis-dichlorobis(ethylenediamine)cobalt(III) perchlorate, [Co(en) 2 Cl 2 ]ClO 4 , is purple.

Trans-dichlorobis(ethylenediamine)cobalt(III) chloride, [Co(en) 2 Cl 2 ]Cl, is green.

NN

Co

Cl

Cl

N

N

N

N

+

cis-isomer

Co

Cl

N

Cl

N

N

N

+

trans-isomer

Complexes involving bidentate ligands, such as ethylenediamine, can also exhibit this

kind of isomerism, as shown in Figure 25-2.

Octahedral complexes with the general formula MA 3 B 3 can exhibit another type of

geometric isomerism, called mer–facisomerism. This can be illustrated with the complex

ion [Pt(NH 3 ) 3 Cl 3 ]
(see Table 25-2). In one isomer the three similar ligands (e.g., the Cl

ligands) lie at the corners of a triangular face of the octahedron; this is called the facisomer

(for facial).

In the other isomer, the three similar ligands lie in the same plane; this is called the

merisomer (so called because the plane is analogous to a meridian on a globe).