Catalytic property complex formation of block element.

Catalytic property.

Most of the transition metals and their compound exhibit catalytic property.They can catalyse both organic and inorganic reaction. The ability to exhibit property is due to the following reason as discussed as below.

1. Formation of intermediate.
2. Interstitial compound.
3. By adsorption.

1. Formation of intermediate.(Variable oxidation state).

Most of the catalyst reaction, proceed by the formation of intermediate.In order to exhibit catalyst property, the transition metal or their compound should form intermediate with reactant molecule.In order to form intermediate compound the transition metal should exhibit variable oxidation ststeSince, most of the transition metal exhibit variable oxidation state they can form the variety of intermediate with different reacting molecule by changing their oxidation state. That’s why only transition metal exhibit catalyst property.

2. Interstitial compound (Absorption).

Transition metal has a unique ability to form an interstitial compound. Transition metal can be absorbed reactant molecule to form the interstitial compound.By doing so reactant molecule is activated and Hence, rate of reaction increased.

By adsorption.

The transition metal provides a large surface area of reaction. The reactant molecule adsorbed on the surface of a catalyst.By doing so the bond in reactant molecule become weak and hence, the rate of reaction increased.

Some of the examples as discussed as below listed.

1. Iron is used as the catalyst in the manufacture of ammonia by Haber's process.
2. V₂O₅ has used as catalyst the manufacture of sulphuric by the contact process.
3. TiCl₄ is used as a catalyst in polymerization od ethane into polyethene.
4. TiCl₃ is used as a Ziegler-Natta catalyst in the production of polyethene from ethane.
5. Pt/Rn is used in Ostwald process during the manufacture of nitric acid.
6. Ni is used in Reppe synthesis in the polymerization of alkynes.
7. CuCl₂ is used Deacon process for making Cl₂ from HCl.
8. Pt used in 3 stages converts for cleaning car exhaust fumes.
9. MnO₂ is used as the catalyst to decompose KClO₃ in the production of oxygen.
10. Pd is used for the hydrogenation of unsaturated compound.

Give reason .

The lowest oxide of a transition metal is the basis whereas the highest oxide is usually acidic.

With an increase in oxidation state of given transition metal, the covalent character of its compound increased. The increased in covalent character also increase the acidic character of the compound. Thus the compound of given transition metal become more and more acidic as oxidation state of transition metal increases.

Thus, the oxide of the given metal in lower oxidation state are basic, those of the metal in intermediate oxidation state are amphoteric while those of the metal in higher oxidation states are acidic.

A transition metal usually exhibits higher oxidation state in its fluorine than in its iodides.

In higher oxidation state, the transition metals have greater charge and smaller size and hence, behave as the hard acid. F^- ion is hard to base and I^- is the soft base.According to HSAB principle, a hard acid will prefer to combine with hard base and soft acid will prefer to combine with a soft base. So, transition metals having higher oxidation state (hard acid) prefer to combine with Fluorine ion (hard base) than iodide ion (soft base).

For example.TiF₄ exist as stable compound as TiI₄ does not exist.

The halide becomes more covalent with increasing oxidations state of the metal and are susceptible to hydrolysis.

With the increased in the oxidation state of the metal, a charge on metal increased and size becomes decreased. According to Fajan’s rule small highly charged ions favours the formation of covalency and hence induce more covalent character. Thus, halides become more covalent with increasing oxidation state of the metal.

Thus the compound of a given transition metal in lower oxidation state are ionic while those of same metals in higher oxidation states are covalent.eg, VCl₂ is ionic, VCl₃ is less ionic while VCl₄ is covalent.

In higher oxidation states of metal ions heaving greater charge and smaller size. We know that smaller the size of the metal ion greater will be the degree of hydration. Hence, higher realised are more susceptible to hydrolysis.

Complex formation.

The transition metal and their cation have a great tendency to form the complexes with several ligands are called coordination compounds. The d-block have marked stability to form complexes than s and p block elements. They form the complexes with several ligands such as neutral molecules NH₃ , H₂O, CO etc. or with anions such as CN^-, Cl^-, SCN^- etc. These ligands possess the lone pair of electrons which they donate to the transition metal cations in the formation of the complex compound.

The tendency of transition metal cations to form the complexes is due to the following reason.

1. The presence of vacant d-orbital.
2. High charge density.
3. Variable oxidation state.
4. Transition metal has the ability to form both sigma and pi bonds with ligands.

1. The presence of vacant d-orbitals.

Transition metal has vacant d orbitals at the low energy level for proper hybridization.So, it can accommodate a lone pair of electron donated by ligands to form the complex compound.

2. High charge density.

Transition metal has relatively small size and has the high charge I,e this ion have high charge density .Therefore, transition metal can exert the strong electrostatic force of attraction of the ligands to form the complex compound.

3. Variable oxidation state.

The transition metal is capable of showing the variable oxidation state which in turns help in complex formation.

4. Transition metal heaving ability to formation of both sigma and Pi bonds,

In complex compounds, there is the coordinate bond between metal and ligands.Hence, complex compounds are also called the coordinate compound.The complex compound contains central coordinate sphere for example. [Fe(CN)₆]^4-

Stability of Chelate.

The stability of chelate is due to the following reason.

1. The crystal lattice of the host compound should contain cavity of appropriate size.
2. The size of the guest compound should be such that it should fit in the cavity of the host component.The guest molecules are tightly held in the cavities of the host molecules.
3. The guest molecules within the cages are at maximum potential energy [Note;guest ‘can escape from the ‘host’ only when the forces holding the molecule cages together are overcome. This can be achieved by the following reason.
4. By heating the crystal.
5. By dissolving them in a suitable solvent such as alcohol.

Reference:

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.