(a) What is meant by the term lanthanoid contraction? What is it due to and what consequences does it have on the chemistry of elements following lanthanoids in the periodic table?
(b) Explain the following observations:
(i) Although Co^{2 +} ion appears to be stable, it is easily oxidised to Co^{3 +} ion in the presence of a strong ligand.
(ii) The $E_{m^{2+}/M}^0$ value for manganese is much more than expected from the trend for other elements in the series.

Dear Student,
(a)  Lanthanoid contraction  is a characteristic of lanthanide series. Also seen in 4d and 5d transition series.The decrease in atomic radii of 4d and 5d transition metals as against the expected increase is said as Lanthanoid contraction.It is the cumulative effect of contraction of lanthanides .
Lanthanoid contraction occurs because as we move  from 3d to 4d the atomic number increases and the filling of f-orbitals starts. The f- orbitals are highly penetrating (closer to the nucleus ) and have a less shielding effect therefore the effective nuclear charge on valence electrons increases causing an abrupt decrease in  atomic size leading to lanthanoid contraction.
Due to lanthanoid contraction the chemical properties of 4d and 5d transition series is almost similar. Because of lanthanoid contraction there is a difficulty in separating lanthanides.
(b) Co²⁺ is easily oxidized to Co³⁺ in presence of ligands because crystal field stabilizing energy of Co³⁺ is more  than that of Co²⁺. Co³⁺ has d⁷ configuration and one of the elctron goes to eg set 
CFSE for Co³⁺ = -2.4${∆}_{\circ }+3P$
CFSE for Co²⁺=$-1.8{∆}_{\circ }+3P$

ii)The  $E_{\raisebox{1ex}{$M{n}^{2+}$}\!\left/ \!\raisebox{-1ex}{$Mn$}\right.}^{\circ }$ is higher as compared to others in the series because of the higher stability of half filled d subshell  in case  of Mn^​2+.
Regaeds