Question

# Explain the following:(i) Why do transition metal ions possess a great tendency to form complexes?(ii) The paramagnetic character in 3d-transition series elements increases up to Mn and then decreases.

Hint: (i) The transition metals are the members of d-block elements. They have partially filled d-subshell which can give rise to cationic species.
(ii) The magnetic properties of transition elements decrease with the decrease in the number of unpaired electrons.

(i) -In the periodic table, the elements of groups 4 to 11 are recognized as transition metals.
-Transition metals either as an element or an ion show good catalytic properties.
-Transition metals form a large range of colored complexes because of the following reasons-
(a) They show variable oxidation states. The oxidation states of some common transition metals are given below-

 Sc Ti V Cr Mn Fe Co Ni Cu Zn +2 +2 +2 +2 +2 +2 +2 +2 +1 +2 +3 +3 +3 +3 +3 +3 +3 +3 +4 +4 +4 +5 +6 +6 +6 +7

(b) They have a small size and high ionic charge. In transition metals, the number of electrons is increasing as we move across a period, and thus, there is more pull of these electrons towards the nucleus. Transition metals have a charge equal to its oxidation state which can vary from +1 to +7.
(c)Transition metals have unstable electrons in their outer orbitals hence they can readily lose electrons than other elements to form complexes.
(d)Transition metals have partially filled d-orbitals for the bond formation. The d-orbital in total can accommodate 10 electrons which makes it possess a great tendency to form complexes.

(ii) -Based on the behavior of compounds in a magnetic field, substances are classified as paramagnetic, diamagnetic, and ferromagnetic.
-The substances which are strongly repelled by the applied magnetic field are called diamagnetic substances.
- The substances which are attracted by the applied magnetic field are called paramagnetic substances.
-The substances which are strongly attracted by the applied magnetic field are called ferromagnetic substances.
-Most of the transition elements exhibit paramagnetic behavior.
-Transition elements have unpaired electrons in (n-1)d orbitals which are responsible for the magnetic properties of transition elements. As we move from left to right along transition elements, the number of unpaired electrons increases from one to five, and hence there is an increase in paramagnetic behavior.
- In the 3d transition series from Sc to Mn, the number of unpaired electrons increases, and hence paramagnetic character increases. After Mn, the pairing of electrons starts and the number of electrons decreases, and hence a decrease in paramagnetic property is observed.

Note: Transition metals have an unparalleled tendency to form more coordination complexes as compared to s block elements and p block elements. There are many important catalysts formed by transition metals for various reactions like $TiC{{l}_{3}}$ (used as Ziegler – Natta catalyst), $FeS{{O}_{4}}$ and ${{H}_{2}}{{O}_{2}}$ (Fenton’s reagent) and many more.