Question

Explain the variation in `"E"_ ("M"^(3+)//"M"^(2+))^0` 3d series.

Answer 1

Transition metals in their high oxidation states tend to be oxidising. The standard reduction potential for the M³⁺/M²⁺ half cell gives the relative stability between M³⁺ and M²⁺.

Reaction	Standard reduction potential (V)
\[\ce{Ti^3+ + e^- -> Ti^2+}\]	−0.37
\[\ce{Y^3+ + e^- -> V^2+}\]	−0.26
\[\ce{Cr^3+ + e^- -> Cr^2+}\]	−0.41
\[\ce{Mn^3+ + e^- -> Mn^2+}\]	+1.51
\[\ce{Fe^3+ + e^- -> Fe^2+}\]	+0.77
\[\ce{Co^3+ + e^- -> Co^2+}\]	+1.81

The negative values for titanium, vanadium and chromium indicate that the higher oxidation state is preferred.

The high reduction potential of M³⁺/M²⁺ indicates Mn²⁺ is more stable than Mn³⁺.