Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

321

pairs on the four nitrogen atoms. The metal is in the center of the large planar ligand. Metal porphyrin complexes, which are common in biochemistry, can differ because of the metal in the center or because of the identity of the R groups on the porphyrin ligand. One of the best known metal porphyrins is

heme

, a porphyrin with Fe

2+ in the center. Heme is

part of the oxygen carrying protein in blood. Another common example is chlorophyll, the light-gathering green pigment in plants. Chlorophyll is a porphyrin

derivative

with Mg

2+ at

the center.

The

ethylene

diamine

tetra

acetate ion, or simply EDTA, can coordinate to metals at six

sites, so it is hexadentate.

It bonds to metals using the lone

pairs on the two nitrogen atoms

and the four oxygen atoms that carry the nega

tive charge (Figure 14.2c). The EDTA ligand

can wrap itself around a metal ion and occupy all

of the sites of an octahedral ion (Figure

14.2d). This property is extremely useful fo

r coordinating to heavy metals and removing

them from the environment. For example, heavy metal ions, such as Pb

2+ and Hg

2+

, are

poisonous because they bind to proteins and cause the protein to stop functioning properly. Na

[Ca(EDTA)] is prescribed for heavy metal poisoning because the EDTA binds so 2

strongly to the Pb

2+ and Hg

2+

ions that they displace the Ca

2+ ion from Ca(EDTA)

2-. Once

bound to the EDTA, the heavy metals can no longer bind to proteins and are passed from the body. EDTA bonds more strongly to Pb

2+

and Hg

2+ than to Fe

2+

, so the beneficial Fe

2+

is not removed by this treatment.

14.2

THE d ORBITALS AND LIGAND FIELDS Recall from Chapter 2 that the orbitals of a subl

evel all have the same energy. Thus all five

d orbitals have the same energy. However, that is true only for

a free atom or monatomic

ion.

The situation is quite different for a metal

coordinated to ligands. If the six ligands in

an octahedral coordination geometry are assumed to lie on the x, y and z axes as shown in Figure 14.3, then the five d orbitals of the metal fall into two groups:

one group of orbitals

is directed along the bonding axes, while the

other is directed between the metal-ligand

bonds

. The z

2 orbital is directed

along

the z-axis, and the x

2 -y

2 orbital

along

the x- and y-

axes, while the xy, xz, and yz orbitals lie

between

the axes.* The six ligands are Lewis

bases, and each ligand approaches the metal with a

lone pair to be used to form a covalent

bond. The lone pairs on the ligands generate an electric field, called the

ligand field

.

Interaction with the ligand field raises the energies of the z

2 and x

2 -y

2 orbitals above that

of the xy, xz, and yz orbitals (Figure 14.4). That is, the energies of the d orbitals that lie

HN^2

HCCH^2

2 NH

2

N

N

N

N

C

C C

C

C CC

C

C

CC C

C

C

C C

C

C

C C

R'

R^8

R^2

R'

R^3 R^4

R'

R^5

R^6

R^7

R'

R^1

HC^2

N

CH^2

CH^2

H^2 C

N CH

H^2 C 2

O

O

O

O

O

O

O

O

(a) eth

ylenediamine (en) (b) porphyrin

(c) ethylenediaminetetraacetate ion (EDTA)

N O

N O

O M O

O

O

O

O

(d)

an empty d orbital isused by the metalto form the metal

ligand bonds

M

M

Figure 14.2 Three common chelating ligands a) A metal is chelated with ethylenediamine. b) A square planar metal porphyrin (the R-groups can be varied). c) EDTA is a very strong chelating agent t

hat can coordinate at six positions. d) The

coordination of EDTA to a metal.

y

x

z

Figure 14.3 Coordinate system used to describe the bonding in six- coordinate metals

* The d orbitals are designated as either a subscripted d (d

2 , dz

(^2) x-y
2 ,
dxy
, d
and dxz
) or by the orbital designation without the d (zxy
2 , x
2 -y
2 ,
xy, xz, and yz). We use the latter method in this text for ease of reading.
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