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Platinum(IV) is perfectly stable in water as its hexachlorido complex, and so is palladium(IV). However, nickel(IV) is not stable in water even in its hexafluorido complex- the exchange-inertness of hexafluoridonickelate(IV) (which is low-spin despite the strong pi-donor property of fluoride; nickel(IV) is "just too high in oxidation state" to admit a high-spin configuration) does not reduce the strong oxidising ability of the central nickel(IV).

My initial guess was that hexaamminenickel(IV) and/or hexacyanidonickelate(IV) would be stable in water, since the otherwise hydrolytically unstable cobalt(III) can be stabilised that way, but I could not find any information about the hydrolytic stability of either ion. So I'm asking it here, just in case stronger sigma-donors than ammonia/cyanide are necessary.

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Nickel may be oxidized from nickel(II) to nickel(IV) with diacetylpyridine dioxime ligands[1] and also in polyniobate complexes[2], in the latter case using hypobromite as the oxidizing agent. The latter reference also notes a Ni(IV) molybdate complex.

References

1. Everett I. Baucom and Russell S. Drago. "Nickel(II) and nickel(IV) complexes of 2,6-diacetylpyridine dioxime". J. Am. Chem. Soc. 1971, 93, 24, 6469–6475. https://doi.org/10.1021/ja00753a022

2. Charles M. Flynn Jr. and Galen D. Stucky."Heteropolyniobate complexes of manganese(IV) and nickel(IV)". Inorg. Chem. 1969, 8, 2, 332–334. https://doi.org/10.1021/ic50072a029

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