Why are `Mn^(2+)` compounds more stable than `Fe^(2+)` compounds towards oxidation to their +3 state ?

Why are Mn^(2+) compounds more stable than Fe^(2+) towards oxidation to their +3 oxidation states ?

Fe^(3+) is more stable than Fe^(2+) . Explain.

In its compounds, oxygen can show oxidation state of

Why are Ni^(2+) compounds thermodynamically more stable than Pt^(2+) compounds whereas Pt^(4+) compounds are relatively more stable than Ni^(4+) compounds ?

Why are haloarenes more stable than haloalkanes ?