What is ionisation enthalpy? Explain why Ni(II) compounds are more stable than Pt(II) compounds while Pt(IV) compounds are more stable than Ni(IV) compounds

Fe^(3+) compounds are more stable than Fe^(2+) compounds because

Fe^(3+) compounds are more stable than Fe^(2+) compounds because

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

Find the true statement/statements a) In a given period Delta_(i)H_(1) is the lowest for an alkali metal and highest for noble gas. b) Cs has the lowest and He has the highest Delta_(i)H_(1) among all elements. c) Lowest Delta_(i)H_(2) among the elements of third period is for Mg. d) The metal with the highest Delta_(i)H_(1) is mercury. e) Ni (II) compounds are thermodynamically more stable than Pt(II) compounds while Ni(IV) compound are less stable than Pt(IV) compounds.

Ni (II) compounds are thermodynamically more stable with Pt(II) compounds.