CHEMISTRY TEXTBOOK

the compound appears blue. The ions having
no unpaired electrons are colorless for example
Cu⊕(3d^10 ); Ti^4 ⊕(3d^0 ). Table 8.8 enlists colours
of 3d transition metal ions.

Table 8.8 Colour of 3d transition metal ions

Ion Outer electronic configuration Number of unpaired electrons Colour

Sc^3 ⊕ 3d^00 Colourless

Ti^3 ⊕ 3d^11 Purple

Ti^4 ⊕ 3d^00 Colourless

V^3 ⊕ 3d^22 Green

Cr^3 ⊕ 3d^33 violet

Mn^2 ⊕ 3d^55 Light pink

Mn^3 ⊕ 3d^44 Violet

Fe^2 ⊕ 3d^64 Pale green

Fe^3 ⊕ 3d^55 Yellow

Co^2 ⊕ 3d^73 Pink

Ni^2 ⊕ 3d^82 Green

Cu^2 ⊕ 3d^91 Blue

Cu⊕ 3d^100 Colourless

Zn^2 ⊕ 3d^100 Colourless

red orange

620 nm

580 nm

560 nm

490 nm

430 nm

400 nm

800 nm

yellow

blue green

violet

Thus the colour of a transition metal ion

relates to the

1. presence of unpaired d electrons


2. d - d transitions


3. nature of ligands attached to the metal ion


4. geometry of the complex formed by the
   metal ion

Let us see how colour of the transition
metal ion depends upon ligand and geometry
of the complex formed by metal ion.

When cobalt chloride (Co^2 ⊕) is dissolved
in water, it forms a pink solution of the
complex [Co(H 2 O) 6 ]^2 ⊕ which has octahedral
geometry. But when this solution is treated
with concentrated hydrochloric acid, it turns
deep blue. This change is due to the formation
of another complex [CoCl 4 ]^2 which has a
tetrahedral structure.

[Co(H 2 O) 6 ]^2 ⊕ + 4Cl^ [CoCl 4 ]^2 + 6H 2 O

Do you know?

Colour of transition metal ions

may arise due to charge transfer. For

example, MnO 4 ion has an intense purple

colour in solution. In MnO 4 , an electron is

momentarily transferred from oxygen (O) to

metal, thus momentarily changing O^2 to O

and reducing the oxidation state of manganese

from +7 to +6. For charge transfer transition

to take place, the energy levels of the two

different atoms involved should be fairly close.

Colours of Cr 2 O 72 , CrO 4 , Cu 2 O and Ni-DMG

(where DMG = dimethyl glyoxime) complex

thus can be explained through charge transfer

transitions.

8.6.6 Catalytic Properties : Transition metals

and their compounds exhibit good catalytic

properties. They have proven to be good

homogeneous and heterogeneous catalysts.

partly because of their ability to participate in

different oxidation-reduction steps of catalytic

reactions.