Chemistry, Third edition

ELEMENTS OF THE FIRST TRANSITION SERIES 207

transition metal’s ability to exist in different oxidation states allows it to easily

make and break bonds with other species, so acting as an efficient catalyst.

6.The transition metals form complex ions. A complex ion consists of a central

metal ion with molecules or ions attached to the metal ion by coordinate bond-

ing. Some main group metals, such as Al, can form complex ions, but the trans-

ition metals form a much greater variety of complexes. Transition metal

complexes are described in more detail in the next section.

7.Compounds of the transition metals tend to be coloured. Although the d elec-

trons in the atoms of transition metals are degenerate(of the same energy), in

complex ions of the transition metals, the d electrons become split in energy,

some at a lower energy than others. The difference between the energies of the d

electrons is of the same order as the energies of light photons in the visible region

of the spectrum. If white light is passed through a solution of a transition metal

complex, d electrons of lower energy may absorb some of the light and be pro-

moted to a higher energy level, thereby absorbing some of the visible light. The

unabsorbed light is responsible for the colour observed.

Common colours of transition metal ions in aqueous solution are:

Cu^2  sky blue Cr^3  violet

Fe^2  pale green Co^2  pink

Fe^3  yellow brown Mn^2  pale pink

Ni^2  green

Oxidation states of

the transition

metals

Transition metals achieve

their oxidation states first

by the loss of the outer s

electrons and thenby the

loss of the underlying d

electrons, which are very

close in energy.

(i)iiWhat is the electronic

structure of atoms of Mn

in oxidation state 2?

(ii)iWhat is the electronic

structure of atoms of Mn

in oxidation state 3?

(iii)What do you think is the

maximum oxidation

state of Mn? Can you

think of a common ion

that has Mn in this

oxidation state?

Exercise 12N

d-block compounds and their

colours

(i)iWhy is the complex ion [Sc(H 2 O) 6 ]^3 

colourless?

(ii)Why are aqueous solutions of zinc compounds

colourless?

Exercise 12O

Transition metal complexes

In transition metal complexes the ions or groups attached to the metal cation are

calledligands. Copper(II) sulfate solution, for example, contains the complex ion

[Cu(H 2 O) 6 ]^2 in which six water ligands are attached to the central copper(II) ion

by coordinate bonds. This complex is often simply written as Cu^2 (aq).

H 2 O

OH 2

2+

OH 2

H 2 O

H 2 O

H 2 O

Cu