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.