Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

The objectives of Chapter 14 are to:

-^

define ligands and discuss their bonding to metals;

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discuss some common shapes adopted by transition metal complexes;

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explain why the d orbitals in coordinated metals do not all have the same energy;

-^

use the fact that the d orbitals

are at different energies to acco

unt for the rich color of many

transition metal compounds;

-^

distinguish between high spin and low spin metal ions;

-^

describe the possible isomers of octahedral coordination compounds;

-^

describe the function of several transition metal ions in biology; and

-^

demonstrate some of the functions of trans

ition metal compounds as catalysts and as

electronic and magnetic materials.

14.1

LIGANDS AND COORDINATION The molecules or ions that bind to a metal are called

ligands

. Typically, ligands contain

lone pairs that can be used to form cova

lent bonds, so they function as Lewis bases.

Ligands vary in size from small molecules, such as H

O and NH 2

, and simple ions, such as 3

Cl

1-and Br

1-, to very large molecules, such as proteins. The formation of a metal-ligand

bond is called

coordination

, but it can be viewed as a Lewis acid-base reaction between a

metal ion with unfilled d orbitals, and a ligand w

ith a lone pair of electrons. The number of

donor atoms bound to a metal is called its

coordination number

. The most common

coordination numbers of transition metals are

four, five and six. The spatial arrangement

of the ligands around the metal is referred to as the metal’s

coordination geometry

.

Figure 14.1 shows the coordination geometries for the common coordination numbers.

Some ligands contain more than one lone pa

ir capable of coordinating to a metal. If

such a ligand bonds to two diffe

rent metals, it is called a

bridging ligand

. The cyanide ion

is a good bridging ligand because it can bond

to the metal through

the carbon and through

the nitrogen, M-C

≡N-M

’. Ligands that can use two lone pairs to form bonds with metals

are said to be

bidentate,

those using four are

tetradentate,

etc

. If the different lone pairs

of the ligand bind to the same metal, the ligand is called a

chelating

• ligand

. Figure 14.2

shows three common chelating ligands.

Ethylenediamine coordinates through the tw

o lone pairs on the nitrogen atoms, as

shown in Figure 14.2a. Because

it coordinates to the metal with two lone pairs, it is a

bidentate ligand

. The

tetradentate

ligand

porphyrin (Figure 14.2b) binds with the lone

(a)

(b)

(c)

(d)

(e)

Figure 14.1 Common coordination geometries Four-coordinate metals are typically tetrahedral (a) or square planar (b). Five-coordinate metals are usually trigonal bipyramidal (c) or square pyramidal (d). Six-coordinate metals are octahedral (e).

* The word ‘chelate’ is derived from the Greek for ‘crab’s claw’

because the ligand-metal bonds appear to ‘pinch’ the metal like a ‘crab’s claw’.

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