Question

Why ni2+ compound is more stable than pt2+ whereas pt4+ are more stable than ni4+ compound?

Answer 1

Ni2+ compounds have the sum of first two ionisation enthalpies to be lower than that of Pt2+ compounds and are therefore thermodynamically more stable. However, the sum of the first four IE of Pt4+ compounds is lower than that of the first 4 IEs of Ni4+ and is therefore relatively stable. 
hope u got it