5.2        Transition Metals

 

Transition Metals

The ten elements from Scandium to Zinc form the first transition metal series.  They closely resemble each other and are hard, dense, shiny metals with high melting and boiling points.  They readily form alloys and have other properties in common.  Crossing the period from Sc to Zn there is a small decrease in atomic radius and increase in electronegativity and ionisation energy.  Most of the properties of transition metals are related to their electronic structures.

 

 

General characteristics such as incomplete d-shell, metallic nature, variable oxidation states, catalytic action, formation of coloured aqua and other complexes. Application of VSEPR to predict shapes of ions etc. mentioned below.

 

 

Electronic Structure

 

Exercise 1

Write down the electron configurations of the elements Sc to Zn using the ‘electrons in boxes’ notation.

                                 3d                     4s

Sc      [Ar]                                                  

Ti      [Ar]  

V        [Ar]  

Cr      [Ar]  

Mn     [Ar]  

Fe      [Ar]

Co      [Ar]

Ni      [Ar]

Cu      [Ar]

Zn      [Ar]

 

 

Transition elements are characterised by having a partially filled d sub-shell

Sc      [Ar] 3d1 4s2

Ti      [Ar] 3d2 4s2

V        [Ar] 3d3 4s2

*Cr    [Ar] 3d5 4s1

Mn     [Ar] 3d5 4s2

Fe      [Ar] 3d6 4s2

Co      [Ar] 3d7 4s2

Ni      [Ar] 3d8 4s2

*Cu    [Ar] 3d10 4s1

Zn      [Ar] 3d10 4s2

 

*Note that in Cr the arrangement [Ar] 3d5 4s1 with half-filled 3d and 4s sub-shells is more stable than [Ar] 3d4 4s2.

In Cu [Ar] 3d10 4s1 with a completely filled 3d sub-shell and a half-filled 4s sub-shell is more stable than [Ar] 3d9 4s2.

 

Variable Oxidation States

The general electron configuration of a transition metal is [Ar] 3dn 4s2. Once the 4s electrons have been removed, the 3d electrons may then also be removed. The difference in energy between the 3d and 4s electrons is much smaller than that between the 3s and 3p electrons, so a varying number of 3d electrons may be removed. This gives rise to the formation of a number of transition metal ions with different oxidation states .  The common oxidation states are ;

Sc                         3

Ti                2        3        4

V              2     3     4     5

Cr                2          3                                 6

Mn               2        3        4                 6        7

Fe                2        3

Co                2        3

Ni                2        3

Cu      1        2

Zn               2

 

The +2 oxidation state becomes relatively more stable compared to higher oxidation states across the series. This reflects the increasing difficulty of removing the 3d electrons as the nuclear charge increases.

 

Exercise 2

Write ‘electrons-in-boxes’ configurations for the following ions.

                                          3d                   4s

1.                   Sc3+    [Ar]                                                  

2.                 V2+     [Ar]

3.                 Cr3+    [Ar]

4.                 Mn2+   [Ar]

5.                 Fe2+    [Ar]

6.                 Fe3+    [Ar]

7.                 Cu2+    [Ar]

8.                 Cu+     [Ar]

9.                 Ni2+    [Ar]

10.              Zn2+   [Ar]

 

 

 

Exercise 3

Work out the oxidation states of the transition elements in the following ions and compounds:

1.                   MnO4-

2.                 MnO2

3.                 K2MnO4

4.                 Cr2O72-

5.                 K2CrO4

6.                 Cr2O3

7.                 KCrO3Cl

8.                 VO2+

9.                 VOCl2

10.              NH4VO3

 

 

Catalytic Action

The ability of transition metals to exist in various oxidation states makes them important industrial and biological catalysts.

 

V2O5

Contact process for the manufacture of sulphuric acid.

2SO2  +  O2                         2SO3

Fe or Fe2O3

Haber Process for manufacturing ammonia

N2  +  3H2                        2NH3

Pt

Manufacture of nitric acid from ammonia.

4NH3  +  5O2              4NO  +  6H2O

then NO             NO2            HNO3

Ni

Hydrogenation of alkenes

CnH2n  +  H2                       CnH2n+2

 

Trace amounts of transition metals are needed for the catalytic activity of some enzymes e.g.  Cu for cytochrome oxidase (needed for metabolism).

 

Complex Ions

These are composed of a central metal atom or ion surrounded by a number of  oppositely charged anions or neutral molecules called ligands.  The ligands donate lone pairs of electrons into the vacant d-orbitals of the transition metal atom or ion forming dative covalent bonds

 

 

Coloured Solutions

Most transition metal compounds are coloured.  In an isolated atom or ion the 3d orbitals have the same energy (degenerate).  In a complex ion the d orbitals are split due to different overlapping with ligands.

 

 


                                                            2 orbitals of higher energy

 

 


                                                    DE   

 

3 orbitals of lower energy

                                   

Transitions between the two levels will absorb energy of frequency n where DE = hn.  For the transition metals this occurs in the visible part of the spectrum, making the ion coloured.

 

 


absorbance




                  l     violet   blue        green   yellow            red

 

The colour of the ion is complementary to the colour absorbed.  i.e. yellow/ green absorbed -ion appears blue/ violet.  The colour of the ion depends mainly on the transition metal but can be affected by different ligands.

CHANGES OF OXIDATION STATE

 

Preparation of chrome alum by reduction of potassium dichromate and preparation of potassium dichromate by oxidation of a chromium (III) salt.

 

Preparation of chrome alum

Chrome alum has the formula KCr(SO4)2.12H2O (but is probably better represented by the formula [K(H2O)6Cr(H2O)6](SO4)2 ). The hexaaquachromium (III) ion gives the compound a violet colour both when solid and in solution. However if the temperature rises above 60 oC the aqua ligands are replaced by sulphate ions and the solution turns green and becomes difficult to crystallise.

Chrome alum can be prepared by reduction of a chromium (VI) salt (potassium dichromate) to a chromium (III) salt.

Cr2O72-  +  14H+  +  6e-                         2Cr3+  +  7H2O

 

The reducing agent is ethanol which is oxidised to ethanal (temperature below 60 oC). The characteristic smell of ethanal is evident at the end of the reaction.

CH3CH2OH                      CH3CHO  +   2H+  +  2e-

 

Balancing electrons and cancelling gives

Cr2O72-  +  8H+  +  3CH3CH2OH                      3CH3CHO  +  2Cr3+  +  7H2O

orange                                                                                                  violet

Chrome alum is quite soluble in water and hence the volume of water used should be kept to a minimum.

 

Preparation

Dissolve 5.9 g (0.02 mol) of potassium dichromate (VI) in 50 cm3 of hot water in a 250 cm3 beaker. Cool the orange solution and add carefully 5 cm3 of concentrated sulphuric acid. Slowly add 10 cm3 of methylated spirit (an excess) using a dropping funnel. Stir constantly with a thermometer and do not let the temperature rise above 60 oC. The colour of the solution changes from orange to violet. Cover the solution with a watch glass and cool in an ice-bath. When crystallisation is complete (a seed crystal may need to be added), filter off the crystals and wash with a little water. Transfer the crystals to a dry filter paper and dry in air.

 

Preparation of potassium dichromate

Potassium dichromate can be made by the oxidation of a chromium (III) salt by a peroxide in aqueous solution.

 

2Cr3+  +  16OH-                          2CrO42-  +  8H2O  + 6e-

3O22-  +  6H2O  +  6e-                          12OH-

Combining the redox reactions gives

2Cr3+  +  4OH-  +  3O22-                       2CrO42-  +  2H2O

                                                          chromate (VI)

 

The addition of acid to an aqueous solution of yellow chromate(VI) eliminates a water molecule and forms the orange dichromate(VI) ion Cr2O72-.

 

2CrO42-  +   2H+                         Cr2O72-  +  H2O

yellow                                                  orange

 

 Preparation

Dissolve 17 g (0.1 mol) of chromium (III) chloride in 40 cm3 of water in a 250 cm3 beaker. In a 100 cm3 beaker dissolve 17 g of potassium hydroxide in 40 cm3 of water. Add the potassium hydroxide solution to the chromium solution. The green precipitate redissolves forming a green solution. Warm, but do not boil and then add with stirring 60 cm3 of hydrogen peroxide solution.  Boil for a few minutes to destroy the excess peroxide and filter while hot. Transfer the yellow solution to an evaporating basin and boil to reduce the volume by half. Add 5 cm3 of glacial ethanoic acid and filter off the orange crystals. Wash with a little water and dry between filter papers.

 

 

The oxidation of Fe (II) to Fe (III)using chlorine and using H2O2.

 

 

 

The reduction of Fe(III) to Fe (II) using zinc and dilute acid and using sulphites.

 

 

 

Titration of iron (II) with acidified manganate (VII)

COMPLEXES

Complexes understood as consisting of a central metal atom or ion surrounded by a number of ligands, defined as anions or molecules possessing lone pairs of electrons. Coordination number. Spatial arrangement of the bonds from the ligands to the central atom or ion up to six outer electron pairs.

 

Complex compounds

Transition metals form complexes or coordination compounds. These are formed by the coordination of lone pairs of electrons from a donor (called a ligand) to a neutral atom or cation (called an acceptor), which has empty orbitals to accept them.

A cation may form a complex with a neutral molecule,  [Cu(NH3)4]2+

or with an oppositely charged ion, [CoCl4]2-.

An atom may form a complex, Ni(CO)4.

 

The charge remaining on the central atom or ion when the ligands are removed is the oxidation number of the metal in the complex. The coordination number is the number of atoms forming coordinate bonds with the central atom or ion : 2,4 and 6 are common.

 

 

The geometric isomerism of square planar platinum complexes e.g. cisplatin. The use and mode of action of cisplatin as an anti-cancer drug.

 

Colours in solution of the hexa-aqua complexes of Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+. The use as qualitative detection tests of the formation of the formation of precipitates of the hydroxides of these metals with NaOH (aq) and NH3 (aq) and where appropriate their subsequent dissolution. Colours, formulae and use in qualitative detection tests of [Cu(NH3)4(H2O)2]2+, [CoCl4]2-, [Fe(CN)6]3-, [Fe(CN)6]4-, [Fe(SCN)(H2O)5]2+, [Ag(NH3)2]+, [Ni(NH3)6]2+.

 

The hydroxides of transition metals are precipitated from solutions of the metal ions by the addition of hydroxide ions. The colour of the precipitate can often be used to identify the metal present. All the precipitates are gelatinous due to hydration. Some form complex ions with ammonia.

Ion

Colour

NaOH (aq)

Dil. NH3 (aq)

Few drops

Excess

Few drops

Excess

[Cr(H2O)6]3+

violet

Cr(OH)3

green/grey ppt.

[Cr(OH)6]3-

 

Cr(OH)3

green/grey ppt.

[Cr(NH3)6]3+

violet soln.

[Mn(H2O)6]2+

pale pink

Mn(OH)2

white ppt.

(rapidly turns brown)

 

Mn(OH)2

white ppt.

(rapidly turns brown)

 

[Fe(H2O)6]2+

green

Fe(OH)2

green ppt.

 

Fe(OH)2

green ppt.

 

[Fe(H2O)6]3+

yellow

Fe(OH)3

rust ppt.

 

Fe(OH)3

rust ppt.

 

[Co(H2O)6]2+

pink

Co(OH)2

blue ppt.

[Co(OH)4]2-

 

Co(OH)2

pink ppt.

 

[Ni(H2O)6]2+

Blue-green

Ni(OH)2

green ppt.

 

Ni(OH)2

green ppt.

[Ni(NH3)6]2+

green soln.

[Cu(H2O)6]2+

blue

Cu(OH)2

blue ppt.

 

Cu(OH)2

blue ppt.

[Cu(NH3)4]2+

deep blue soln.

 

 

 

 

 

 

 

 

 

Qualitative analysis tests

 

Colours, formulae and use in qualitative detection tests of [Cu(NH3)4(H2O)2]2+, [CoCl4]2-, [Fe(CN)6]3-, [Fe(CN)6]4-, [Fe(SCN)(H2O)5]2+, [Ag(NH3)2]+, [Ni(NH3)6]2+, [CuCl4]2-, [Co(NH3)6]2+

 

·        Copper (II)

Addition of excess ammonia to a solution of Cu2+ ions produces a deep blue solution by ligand replacement.

 

[Cu(H2O)6]2+  +  4NH3                          [Cu(NH3)4(H2O)2]2+  +  4H2O

pale blue                                            deep blue solution  

 

[Cu(H2O)6]2+  +