`color{green}(text(Definition ))` : Complex compounds are those in which the metal ions bind a number of anions or neutral molecules giving complex species with characteristic properties.

● A few examples are : `color{red}([Fe(CN)_6]^(3–), [Fe(CN)_6]^(4–), [Cu(NH_3)_4]^(2+))` and `color{red}([PtCl_4]^(2–))`.

`=>` The transition metals form a large number of complex compounds.

● This is due to the comparatively smaller sizes of the metal ions, their high ionic charges and the availability of `color{red}(d)` orbitals for bond formation.

`=>` The transition metals and their compounds are known for their catalytic activity.

● This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes.

● Examples : (i) Vanadium(V) oxide (in Contact Process)

(ii) finely divided iron (in Haber’s Process), and

(iii) nickel (in Catalytic Hydrogenation)

`=>` Catalysts at a solid surface involve the formation of bonds between reactant molecules and atoms of the surface of the catalyst (first row transition metals utilise `color{red}(3d)` and `color{red}(4s)` electrons for bonding).

● This has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowering).

● Also because the transition metal ions can change their oxidation states, they become more effective as catalysts.

● For example, iron(III) catalyses the reaction between iodide and persulphate ions.

`color{red}(2I^(-) +S_2O_8^(2-) → I_2+2SO_4^(2-))`

An explanation of this catalytic action can be given as :

`color{red}(2Fe^(3+) +2I^(-) → 2Fe^(2+) +I_2)`

`color{red}(2Fe^(2+) +S_2O_8^(2-) → 2Fe^(3+) +2SO_4^(2-))`

`color{green}(text(Definition ))` : Interstitial compounds are those which are formed when small atoms like `color{red}(H)`, `color{red}(C)` or `color{red}(N)` are trapped inside the crystal lattices of metals.

● They are usually non stoichiometric and are neither typically ionic nor covalent.

● Example : `color{red}(TiC, Mn_4N, Fe_3H, VH_(0.56))` and `color{red}(TiH_(1.7))`, etc.

● The formulas quoted do not, of course, correspond to any normal oxidation state of the metal.

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

`=>` The principal physical and chemical characteristics of these compounds are as follows :

(i) They have high melting points, higher than those of pure metals.

(ii) They are very hard, some borides approach diamond in hardness.

(iii) They retain metallic conductivity.

(iv) They are chemically inert.

`color{green}(text(Definition ))` : An alloy is a blend of metals prepared by mixing the components.

● Alloys may be homogeneous solid solutions in which the atoms of one metal are distributed randomly among the atoms of the other.

● Such alloys are formed by atoms with metallic radii that are within about `15` percent of each other.

● Because of similar radii and other characteristics of transition metals, alloys are readily formed by these metals.

● The alloys so formed are hard and have often high melting points.

● The best known are ferrous alloys : chromium, vanadium, tungsten, molybdenum and manganese are used for the production of a variety of steels and stainless steel.

● Alloys of transition metals with non transition metals such as brass (copper-zinc) and bronze (copper-tin), are also of considerable industrial importance.