Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

325

14.3

ISOMERS When multiple types of ligands are attached

to the same metal center, their arrangement

about the metal strongly influences the properties of the complex. Complexes that differ only in the arrangement of the ligands around the metal are known as geometric isomers. This is a stereoisomer similar to those we

encountered for organic compounds in Chapter

13. The chemical and physical properties of

geometric isomers can be very different.

As shown in Figure 14.6, two ligands can be either

cis

(next to one another) or

trans

(opposite one another). Although the three

cis

structures shown in Figure 14.6a may look

different, they are all the same isomer; each ca

n be rotated such that it is superimposable

with either of the other two. The same is true of the three

trans

structures shown in Figure

14.6b. There are only two isomers of a compound of this type, one

cis

isomer and one

trans

isomer. There are only two ways in which three liga

nds can be situated in an octahedral

geometry (Figure 14.7). One is said to be

meridial

(mer

) because the three ligands lie on

an arc of a circle (the meridian) going around the octahedron. The other is called

facial

(fac

) because the three ligands define one of

the faces of the octahedron (Figure 14.7c).

Example 14.3 a) Draw all of the isomers of square planar [Pt(NH

) 32

Cl

]. 2

As in the case of octahedral complexes,

the two chlorine atoms or the two ammonia

molecules of a square planar complex

can be either next to one another (

cis

) or opposite

one another (

trans

), as shown in Figure 14.8. It may look like there are other ways to

arrange the four groups around the platinum, but

the other structures that can be drawn

can all be arrived at by simply rotating one of the above two isomers.

b) Draw the isomers of square planar [PtCl

(NH 3

)] 3

1-.

Only one structure can be drawn for [Pt(NH

)Cl 3

1-] 3

because it does not matter which of the

four positions is occupied by the one ammonia molecule.

M

LLL

Cl

Cl L

M

LClL

Cl
L L

M

Cl L

L L

L Cl

M

LLL

L

Cl Cl

M

Cl L

L Cl

L L

M

LClCl

L

L L

(a) (b)

Figure 14.6

cis

and

trans

isomers of an octahedral metal

a) The two chloride ions are

cis

. b) The two chloride ions are

trans

. L

is used to represent unspecified ligan

ds that are not chloride ions.

M

L L

L Cl

Cl Cl

M

Cl L

L Cl

Cl L

M

LClCl

Cl

L L

M

LClL

Cl

Cl L

M

LClL

Cl

L Cl

M

Cl L

Cl L

L Cl

(a) (b)

LCl

L

LCl

Cl

(c)

face of octahedron

Figure 14.7

fac

and

mer

isomers of an octahedral metal

a) The three chloride ions are meridial. b) The three chloride ions are facial. c) Three facial chloride ions

define one of the eight faces of the

octahedron. L represent

s unspecified ligands.

H^3

NPtCl

Cl NH

3

H^3

NPt

Cl Cl

NH

3

cis

trans

Figure 14.8

cis

and

trans

isomers of Pt(NH

) 32

Cl

(^2)
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