The transition element in the periodic table contains the elements in the groups 3 to 12 in which the d orbitals are progressively filled in each of the four long periods. The name transition it self declare they form a chemical bridge between the highly electropositive s-block element and the highly electronegative p block element. Transition elements are mostly metals and have distinct chemical properties.
Transition metal various in the chemical reactivity to a considerable extent. Gold and platinum are considered as noble metal because they are unreactive chemically where as chromium and Manganese are highly reactive. There are mainly three series of the transition metals, 3d series which starts from Scandium and ends in Zinc (Sc to Zn), 4d series starts from Yittrium and ends in Cadmium (Y to Cd) and 5d series starts from Lanthanum and ends in Mercury (La to Hg,) Because of increase in nuclear charge which accompanies the filling of the inner d orbitals, there is an increase in ionization enthalpy along each series of the transition elements from left to right.
Complex ions and ligands
Complex ion is a structure in which it contain a central atom(usually metal) and surrounded by array of molecules or anions called ligands. The central metal atom and the ligand are joined together by coordinate bond.
Some of the properties of transition elements are
Oxidation states of transition metals
- Oxidation state
- Formation of colored compounds
- Catalytic properties
One of the main property of the transition element are the variable oxidation number they exhibit variable oxidation state in their compound. Manganese can exhibit from +2 to +7. This makes the chemical properties of transition element interesting. Because they can act as good reducing agent as well as good oxidizing agent. For example KMnO4 is a good oxidizing agent since Mn is in the oxidation number of +7 where as the Mn metal which is a metal in the oxidation number 0 is good reducing agent getting itself oxidized to +3 state. The elements which give the greatest number of oxidation states occur in or near the middle of the series. Transition metal ions with charge greater than +3 cannot exist in aqueous solution.
Formation of colored complexes
Transition metals vary widely in their chemical reactivity. Many of them are sufficiently electropositive to dissolve in mineral acids. Metal in the first transition series with the exception of copper are relatively more reactive and are oxidized by dilute acids.
Most of the transition elements form colored compounds because of the existence of electronic transitions. Electronic transitions are of two types.
- Charge transfer transitions
- Charge transfer transitions
The transition of electrons preferably from the ligand orbital to metal orbital producing the ligand to metal charge transfer transition (L â M). If the metal posses high oxidation state then this transition can easily occur. The color of permanganate, chromate and dichromate are due to L â M charge transfer transitions.
This kind of transition occurs when electron is transferred from one d-orbital to the other. All the d-orbitals are not having the same energy. This kind of transition energies occurs in visible band giving rise to strong color for transition metal complexes.
The catalyst may increase the reaction rate or enable the reaction to process at lower temperature. If a catalyst is used in order to increase the reaction rate then the activation energy will be less for the reaction. It follow different transition state and alternative reaction mechanism. The catalyst may involve in the reaction and increase the reaction rate and separates out at the end of the reaction.
Due to the existence of variable oxidation state many of the transition metals posses catalytic properties. They are known for the homogeneous and heterogeneous catalyst. For example, Vanadium is used as its oxide in contact process. Iron is used in Haber's process in finely powdered form and nickel is used in catalytic hydrogenation.