Chemistry FORMATION OF COMPLEXES,ALLOY AND INTERSTETIAL COMPOUND & CATALYTIC PROPERTIES

Formation of Complexes :

By virtue of their small size, comparatively high nuclear or ionic charge and availability of vacant `d`-orbitals of suitable energy, these metals exert strong electrostatic attraction on the ligands. The species formed on interaction of metal and the ligand (or ligands) is known as a complex.

The transition metal ions form complexes because of the following reasons :

(a) Their small cation size.

(b) High effective nuclear charge.

(c) Availability of vacant (`n-1`) `d`-orbitals of appropriate energy.

(d) The structure commonly found in such complex are linear (i.e. co-ordination number, `C.N.=2`), square planer (`C.N. =4`), tetrahedral (`CN = 4`) or octahedral (`CN = 6`).

(e) Cobalt form more complex than any other elements.

`Co^(3+) + 6 NH_3 -> [Co (NH)_6]^(3+)`

`Fe^2 + 6CN^(-) -> [Fe (CN)_6]^(4-)`

`Co^(3+) + 6 H_2O -> [Co(H_2 O)_6]^(3+)`

See given table for more examples.

Formation of Interstitial Compounds :

(i) Small non metallic atom such as `H`, `B`, `C`, `N` etc. are able to occupy interstitial space of the lattice of the `d`-block elements to form combinations which are termed interstitial compounds.

(ii) These are non-stoichiometric in nature and do not follow the common rule of valency.

(iii) These interstitial compounds have similar chemical properties as the parent metal but differ appreciably in their physical properties such as density, hardness and conductivity.

Alloy Formation :

(i) Transition metals form a large number of alloys.

(ii) `d`-block elements are quite similar in atomic size, the atom of one metal can substitute the atoms of other metal in its crystal lattices.

(iii) Thus, on cooling a mixture solution of two or more transition metals, smooth solid, alloys are formed.

(iv) Alloy containing mercury as one of the constituent elements are called amalgams.

(v) The purpose of making alloy is to develop some useful properties which are absent in constituent element.

Catalytic Properties :

Many transition metals & their compounds have catalytic properties. Some common examples are-

(a) In some cases the transition metals with their variable valency may fonn unstable intermediate compounds.

(b) In other cases the transition metal provide a suitable reaction surface.

(c) Enzymes are catalyst that enhance the rate of specific reactions. Some enzymes require the presence of metal ions as cofactors and these are called metalloenzymes.

(i) `TiCl_3`- Used as Ziegler- Natta catalyst in the production of polyethene.

(ii) `V_2O_5`- Convert `SO_2` to `SO_3` in the contact process for making `H_2SO_4`.

(iii) `MnO_2`- Used as a catalyst to decompose `KClO_3` to give `O_2`.

(iv) `Fe`-promoted iron is used in Haber- Bosch process for making `NH_3`.

(v) `FeCl_3`- Used for making `C Cl_4` from `CS_2 & Cl_2`.

(vi) `FeSO_4` & `H_2O_2`- Used as Fenton's reagent for oxidizing alcohol to aldehyde.

(vii) `Pt`- used as a catalyst in the manufacture of `H_2SO_4`.

(viii) `Ni`- used as a catalyst in the hydrogenation of oils.

(ix) `PdCl_2`- Wacker process for converting `C_2H_4 + H_2O + PdCl_2` to `CH_3CHO + 2HCl + Pd`.

(x) `(Pt)//(PtO)`- Adams catalyst, used for reduction.