Transition Metals

Definitions

• Complex: A metal ion surrounded by ligands.

• Ligand: an electron pair donor i.e. a molecule or ion joined onto the metal ion by a dative covalent bond to the metal.

• Coordination number: The number of atoms directly joined to a transition metal by a coordinate (dative covalent) bond.

Typical ligands

• Unidentate H2O: :NH3 :Cl-

• Bidentate C2O42- H2NCH2CH2NH2

• Multidentate EDTA4-

• Watch out for the chelate effect.

Shapes

• Six-coordinate complexes are octahedral.eg Cu(H2O)6

2+ Co(NH3)63+

• Four-coordinate complexes are usually tetrahedral eg CoCl42- but a few are square planar eg Ni(CN)4

2-.

• Two-coordinate complexes are linear eg AgCl2-.

Colour

• Most transition metal colours are due to d-d electron transitions. The energy gap between the split d-orbitals corresponds to visible light (E=hf).

• Some of the really strong colours are due to charge transfer (such as in MnO4

-).

Changing colour

• Anything that changes the energy difference between the d-orbitals causes a change in colour:

• Oxidation state

• Ligand

• Coordination number

Measuring concentration by colour

A measurement of colour intensity (using a colorimeter) can measure the concentration of a solution.

Sometimes a special complex is formed so that the colour is more intense.

This is known as UV/visible spectroscopy.

eg Fe(H2O)63+ + SCN- Fe(H2O)5(SCN)2+ + H2O

Reaction types

• The reactions of the transition metals come under three headings (but sometimes more than one occurs at the same time!).

• Ligand substitution

• Hydrolysis (the acidity reaction)

• Redox

Ligand substitution

• One kind of ligand is replaced by another.

For example,

Cu(H2O)62+ + 4Cl- → CuCl42- + 6H2O

Hydrolysis (the Acidity reaction)

• One or more hydrogen ions is removed.

For example,

Cu(H2O)62+

(aq) + 2OH-(aq) →

Cu(H2O)4(OH)2(s) + 2H2O(l)

Redox

The oxidation state of the metal is changed by adding or removing one or more electrons

For example,

Co(NH3)62+ → Co(NH3)6

3+ + e-

Titanium

• Mainly known for its chloride, TiCl4, as part of the Kroll process for extraction of titanium from its ores.

• The chloride is covalently bonded.

• Titanium oxide, TiO2 , is used as a white pigment.

Vanadium

• Most stable form is V(IV) eg as VO2+.

• V(V) can be reduced in acidic conditions (eg Zn / HCl)

VO2+ → VO2+ → V(H2O)6

3+ → V(H2O)62+

V(H2O)62+ is unstable in air since it is

oxidised by oxygen.

Chromium

• Cr(H2O)63+ Cr(H2O)3(OH)3 Cr(OH)6

3-

>>>>Increasingly alkaline conditions>>

Cr2O72- CrO4

2-

<<<<Increasingly alkaline conditions<<<<

Manganese

• Mn(H2O)62+ MnO2 MnO4

2- MnO4-

II IV VI VII

v. pale pink brown deep purple

MnO4- is a very strong oxidising agent.

Use acidic conditions. For titrations, make sure you add sufficient sulfuric acid to take Mn all the way to Mn2+. Otherwise you get incomplete reduction (the brown solid, MnO2 is formed, rather than Mn2+).

Iron

• Fe(H2O)62+ is non-acidic in water (green).

• Pure Fe(H2O)63+ is a lilac colour but on

contact with water goes rusty brown.

• Fe(H2O)62+ forms Fe(OH)2 (a green solid)

with NaOH but it goes brown (forming Fe(OH)3 on standing in air.

Cobalt

add ammonia solution add hydrogen peroxide (or air)

Co(H2O)62+ → Co(NH3)6

2+ → Co(NH3)63+

pink brown / yellow orange

CoCl42- Co(OH)2

blue blue-grey

Add NaOH or NH3 (aq)

Add conc

HCl

Nickel

add ammonia solution

Ni(H2O)62+ Ni(NH3)6

2+ green blue

NiCl42- Ni(OH)2 Ni(CN)42-

blue pale green a square planar complex!

Add NaOH or NH3 (aq)

Add conc

HCl

Add NaCN (aq)

Copper

add ammonia solution

Cu(H2O)62+ Co(NH3)4(H2O)2

2+

blue deep blue / violet

CuCl42- Cu(OH)2

yellow-green pale blue

Add NaOH or NH3 (aq)

Add conc

HCl

Silver

Small quantity of NaOH, NH3(aq) excess NH3 (aq)

Ag+(aq) Ag2O(s) Ag(NH3)2

+

Cl-

AgCl(s)

excess Cl-

AgCl2- Ag(S2O3)23- Ag(CN)2

-

NaCN(aq)

Na2S2O3(aq)

Uses of transition metalsand their complexes

• V2O5 is used as a catalyst in the Contact Process (manufacture of sulfuric acid).

• Cis-platin (a platinum complex) is a very effective anti-cancer drug.

• Fe2+ is an important part of haemoglobin.• Ag(NH3)2

+ is Tollen’s reagent (test for aldehydes).

• Ag(S2O3)23- is formed during photographic

processing.• Ag(CN)2

- is used in electroplating.