The Transition Elements

D-block elements are the elements in which the d-subshells are being filled.

i.e. Sc Ti V Cr Mn Fe Co Ni Cu Zn

The transition metals are compounds that form at least 1 stable ion where the compound has an incomplete d subshell. Therefore, Scandium is not a transition metal, because it forms only Sc3+ ions with n d-electrons, and Zn is not a transition metal because it forms only Zn2+ ions with all the 3d electrons present. Transition metals form a transition between the properties of group 1 and 2 to the properties of group 3 and 4.

The general characteristics of the transition metals are :

Electronic structures of the d-block elements:
(Ar) represents 1s2 2s2 2p6 3s2 3p6

Sc - (Ar) 4s2 3d1
Ti - (Ar) 4s2 3d2
V - (Ar) 4s2 3d3
Cr - (Ar) 4s1 3d5
Mn - (Ar) 4s2 3d5
Fe - (Ar) 4s2 3d6
Co - (Ar) 4s2 3d7
Ni - (Ar) 4s2 3d8
Cu - (Ar) 4s1 3d10
Zn - (Ar) 4s2 3d10

The configurations in bold are the ones that might not be expected. They are due to the fact that a full or half-full 3d shell is more stable than a full 4s shell. LEARN THEM!

The d-block elements form +ve ions by losing the 4s electrons first. This leads to the similarities in reactivity. They may then also loose some or all of their 4d electrons. The sizes of the ions are all similar, leading to similar physical properties.

Variable Oxidation Numbers

This is a list of the ONs, and therefore the ions, of the d-block elements. Those in bold are common and stable.

Sc

   

+3

       

Ti

 

+2

+3

+4

     

V

 

+2

+3

+4

+5

   

Cr

 

+2

+3

   

+6

 

Mn

 

+2

+3

+4

(+5)

+6

+7

Fe

 

+2

+3

       

Co

 

+2

+3

       

Ni

 

+2

+3

+4

     

Cu

+1

+2

         

Zn

 

+2

         
In solution in water, complex ions form aqua complexes. The different oxidation states of these ions and the different ligands attached to them allow them to form different colours of complexes.

Bonding in complex ions

The ions of non-transition metals (e.g. sodium) exist in water as discrete ions (i.e. Na+) Some water is attracted by the strong positive charge on the ion and this may cause dipole - dipole attraction, but there is no real bonding, only a loose attraction between the water and the ion.

Transition metal ions, however, will form dative covalent bonds when in solution, with either water molecules, or other molecules and / or ions. These small species attached onto the ion are called ligands. The most common ligands are H2O, NH3, Cl-, CN-, and some organic molecules with N atoms. Lone pairs from these ligands go into empty 3d, 4s and even 4p orbitals. Fill in the ions in the diagrams below:

 

 

ion - the hexa aqua iron (II) ion. This forms an octahedral shape.

 

 

 

Tetra amine Copper (II) ion - ion.

This ion is a dark blue colour.

 

 

Normal copper (II) ions are tetra aqua copper (II) ions - (According to the textbook, there are 6 waters)

Hexa cyano ferrate (II)

The 6 CN- ions change the overall charge from 2+ to 4-.

Some ions may have different isomers, depending on the number of each ligand in the ion. More information on complex ions is given in Chemistry in Context.

Further Properties of Transition Metals

Aqua Complexes

When in solution, the ions of Transition metals form what are known as aqua complexes. These are complex ions with water as the ligands. The different ions form complexes with different colours, as shown in the table below:

Ion

Colour

Copper (II)

Blue

Cobalt (II)

Red

Manganese (II)

Pale pink

Iron (II)

Green

Iron (III)

Orange, yellow or brown

Vanadium (II)

Purple

Vanadium (III)

Green

Vanadium (IV)

Blue

Vanadium (V)

Yellow

Reasons for the coloured ions:

The atom of the metal absorbs light of certain wavelength. This causes electrons to be promoted to a certain energy level. The energy jump here corresponds to the energy of the light that is emitted. In a normal metal atom, the only possible energy jumps are too great for visible light to be affected, and so the solutions appear colourless. However, transition metals are different. When there are ligands present, the 3d level splits up into two levels of energy, with two orbitals at a slightly higher level and three at a lower level. The energy jump when an electron is transferred between two of these levels corresponds to the energy of visible light. This is why the solutions of the ions are coloured.

Reactions with NaOH and NH3:

Transition metal ions are acidic in nature. This is due to a process known as deprotonation. The positive transition metal ion is complexed with 6 H2O molecules. However, the positive charge on these molecules attracts the electron on one of the OH bonds of the water molecules. This releases the H+ ion, which has had its electron removed, as a proton in solution, thus giving the process its name. This leaves, using iron (III) as an example, . The proton would then be attracted to the lone pair on an oxygen atom in a water molecule to for the hydroxonium ion, H3O+.

This leaves us with an acidic solution. However, the process is much accelerated by the addition of an alkali, which liberates OH- ions in solution. This causes increased deprotonation, and the ion forms as many hydroxide ligands as it had positive charges on it at the beginning. This is the hydroxide of that ion, which is invariably insoluble, and so as a result, a precipitate forms. This is the cause of the precipitates when an alkali not in excess reacts with a transition metal solution. However, when the alkali is in excess, other things may happen.

Amphoteric Hydroxides

When an amphoteric hydroxide is formed by the precipitation reaction, in the presence of further NaOH it will react with this to form an anion. This anion will have more hydroxide ions as ligands, and, being negative in charge, will dissolve in water to form a solution once again. The only case in which this happens ion the 4th period transitions is Cr3+.

Ligand Exchange

Ammonia is too weak a base to react with amphoteric hydroxides, but another type of reaction may take place. The ammonia molecules may displace the OH- ions and some of the H2O molecules as ligands, making the ions soluble again. However, the amine complexes formed often have different colours to the aqua and hydroxide complexes that they have replaced. For a list of these reactions see the included table.

The Oxidation States of Vanadium

Vanadium has 4 oxidation states, shown below.

Oxidation State

Ion

Colour

+5

VO2+

Yellow

+4

VO2+

Blue

+3

V3+

Green

+2

V2+

Purple

Usually, when preparing these solutions, the starting point is a solution of NH4VO3(aq). This provides VO2+ ions in solution. This can be reduced to:

+4 using potassium iodide, when sodium thiosulphate is also needed to remove the brown iodine colour.
+3, using tin metal (this is a very slow reaction)
+2, using zinc metal (this is a very fast reaction)

The equations for these reactions are shown below: