Chemistry of transition elements II

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7) Transition elements in redox systems

These transition metals were highlighted in the syllabus. For e.g. Fe3+ can act as a reducing agent while it itself is oxized to Fe2+.

8) Transition metals react with ligands to form complexes
A ligand possess one or more lone pairs of electrons and donates its electrons to the transition metal ion

A complex is formed by the coordination of lone pairs of electrons from a ligand to a cation which has empty orbitals to accomodate them. Transition metal ions have incompletely filled 3d subshells to accomodate the lone pair donated by the ligand.

A coordinate bond is a covalent bond in which the shared paired of electrons is provided by only one of the bonded atoms. In the case of transition ion complexes, the shared paired of electrons is provided by the ligand

9) Ligand exchange
A stronger ligand can displace a weaker ligand from a complex ion.
The order of ligand strength is CN->NH3>Cl->H2O

CO and oxygen are ligands for Fe in haemoglobin
Haemoglobin contains an Fe2+ ion. Oxygen can act as a ligand and form coordinate bonds with Fe2+. This bonding is reversible to allow haemoglobin to release oxygen where it is needed. Carbon monoxide is a stronger ligand than oxygen. It bonds strongly to Fe2+ . This prevents Fe2+ and thus haemoglobin from bonding with oxygen. This prevents the transport of oxygen in the body eventually leading to death.

10) Transition metals can act as catalyst

Transition metals can act as heterogenous catalysts. Reactant molecules are adsorbedon the surface of the catalyst . The 3d electrons of transition metals enable the transition metal to form temporary bonds with reactant molecules, facilitating the breaking of "old" bonds and the formation of "new" bonds leading to the product formation. Examples include iron (catalyst for the Haber process), Ni (catalyst for hydrogenation).

Transition metals can also act as homogenous catalysts due to the variable oxidation states of transition metals.


For example, the oxidation of iodide ions by peroxodisulphate ions is energetically favourable but in reality the reaction takes place very slowly without a catalyst due to the fact that both reactants are negative ions thus they repel each other.

Fe2+ can act as a catalyst for this reaction. First Fe2+ reduces peroxodisulphate ions to sulphate ions and it itself is oxidized to Fe3+.


Subsequently, Fe3+ oxidizes iodide ions to iodine and Fe2+ is regenerated.

11) Transition metal complexes are coloured
In an isolated tansition metal aton, the 3d orbitals are at the same energy level (degenerate) . However in a complex ion , the d orbitals have slightly different energy due to the presence of ligands (d orbital splitting). An electron can be promoted to a higher energy level by absorbing a photon of light. The color of the complex metal ion is the frequencies of light that are not absorbed.



End of chemistry of transition metals
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