= Solution
The more radially extended Pt $5d$ orbitals give strong <metal>–ligand overlap and covalent bonding. This helps compensate the energetic cost of reaching Pt(IV), and the large octahedral <ligand field> splitting strongly stabilizes its low-spin $d^6$ configuration, with a <crystal field stabilization energy> contribution $-2.4\Delta_o$ before separately counting pairing and the full bond energies. Ni $3d$ bonding is less effective in this respect; Pd $4d$ compounds also generally favour the very stable $d^8$ M(II) arrangement more strongly than Pt does. \b[The stronger bonding and large ligand-field stabilization available to Pt(IV) make oxidation to $+4$ more favourable relative to its congeners.] This is a comparative trend, not a claim that Ni(IV) or Pd(IV) complexes cannot exist.
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