Tendency to form Complexes, Formation of Interstitial compounds and Alloys, Catalytic Properties

The transition elements form a large number of coordination complexes. The transition metal ions bind to a number of anions or neutral molecules in these complexes.

The common examples are [Ni(NH₃)₆]³⁺, [Co(NH₃)₆]³⁺ ,  [Fe(CN)₆]³⁺ ,  [Fe(CN)₆]^4-,  [Cu(NH₃)₄]²⁺.

The high tendency of transition metal ions to form complexes is due too :

1) small size of the atoms and ions of transition metals

2) high nuclear charge

3) Availability of vacant d-orbitals of suitable energy to accept lone pairs of electrons donated by other groups (called ligands).

Transition  metals form interstitial compounds with elements such as hydrogen, boron, carbon and nitrogen. The small atoms of these non-metallic elements (H, B, C, N, etc.) get trapped in vacant spaces of the lattices of the transition metal atoms.

They are generally non-stoichiometric and are neither typically ionic nor covalent. The common examples of interstitial compounds of transition metals are TiC, Mn₄N, Fe₃H, TiH₂ etc.

The non-stoichiometric materials are obtained having the composition as TiH_1.7 , VH_0.56 

Because of the nature of their composition, these compounds are referred to as interstitial compounds.

As a result of the filling up of the interstitial spaces, the transition metals become rigid and hard. These interstitial compounds have similar chemical properties as the parent metals but differ significantly in their physical properties particularly, density, hardness and conductivity.

For example: steel and cast iron are hard because of the formation of interstitial compounds with carbon.

Reason : The transition metals can easily accommodate the small non- metallic atoms because of spaces between the metal atoms. These spaces are present because of defects in their structures and existence of variable oxidation states.

The general characteristic physical and chemical properties of these compounds are:

1) They have high melting points which are higher than those of pure metals.

2) They retain metallic conductivity.

3) They are very hard. Some borides have hardness as that of diamond.

4) They are chemically inert.

Many transition metals and their compounds act as good catalysts for various reactions. Of these, the use of Fe, Co, Ni, V Cr. Mn, Pt, etc. are very common.

For example

a) Iron-molybdenum is used as catalyst in the synthesis of ammonia by  Haber’s process.

b) Nickel is used in hydrogenation reactions in organic chemistry.

c) V₂O₅ is used for the oxidation of SO₂ to SO₃ in the Contact process tor the manufacture of H₂SO₄

d) MnO₂ is used to catalyse the decomposition of H₂O₂ solution.

e) Cobalt salts catalyse the decomposition of bleaching powder.

1) The catalytic property of transition metals is due to their tendency to form reaction intermediates with suitable reactants. These intermediates give reaction paths of lower activation energy and, therefore, increase the rate of the reaction. These reaction intermediates readily decompose yielding the products and regenerating the original substance. The transition metals form these reaction intermediates due to the presence of vacant orbitals or their tendency to form variable oxidation states.

2) The transition metal catalysts provide a suitable large surface area on which the reactants may be adsorbed and, therefore, come closer to one another for the reaction. This increases the concentration of the reactants at the catalyst surface and also weakens the bonds in the reactant molecules. Consequently, the activation energy gets lowered.

For example: During the conversion of SO₂ to SO₃, V₂O₅ is used as a catalyst.

Solid V₂O₅ adsorbs a molecule of SO₂ on the surface to form V₂O₄, and the oxygen is given to SO₂, to form SO₃. The divanadium tetroxide is then converted to V₂O₅  by reaction with Oxygen:

3) The transition metal ions can change their oxidation states and become more effective as catalysts.

For example:  iron (iii)  catalyses the reaction between iodide and persulphate ions (S₂O₈^2-) as:

Cobalt salts catalyse decomposition of bleaching powder as cobalt can easily change oxidation state from +2 to +3 as:

For example: the most common known alloys are ferrous alloys. Chromium, manganese, vanadium, tungsten, molybdenum etc. are used to produce variety of steels and stainless steel.

Alloys of transition metals with non-transition metals such as bronze (copper-tin), brass (copper-zinc) are also industrially important alloys.

The transition metals are quite similar in size and, therefore, the atoms of one metal can substitute the atoms of other metal in its crystal lattice. Thus, on cooling a mixture solution of two or more transition metals, solid alloys are formed.