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The chloride ion forms strong anionic complexes with many metals. Well known examples include tetrachlorocuprate(Ⅱ) and tetrachloroaurate(Ⅲ). Many of these complexes are stable in aqueous solution, but most cannot be obtained as an acid hydrate, only as salts with other metals. Some stable fluoride complexes such as hexafluoroantimonate(Ⅴ) or hexafluorosilicate(Ⅳ) do not have chloride equivalents because of hydrolysis(?) while some such as hexachlorotitanate(Ⅳ), trichlorocuprate(Ⅱ), and hexachloromanganate(IV) are moisture sensitive due to oxophilicity, the existence of more stable chloride complexes, and strong oxidative power respectively.

I am specifically asking about what determines which of these chloride complexes have isolable hydronium salts, as as far as I know none of them are isolable as a free acid. I am aware of the hydrates of HICl4, HAuCl4, and H2PtCl6, but aside from their cores being hard-to-oxidize heavy elements (one is a halogen and the other two are noble metals) I do not see the pattern determining isolability. I have not undergone formal education on orbitals and complex coordinate bonds or similar topics, so it would be preferrable if the answer is not too technical.

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Generally speaking, the central atom in an isolated chloro acid has the following characteristics:

  • Late transition or post-transition element group. Elements in earlier groups tend to be oxophilic and preferentially form oxo, hydroxo or aquo complexes.

  • Higher periods, al least fifth and preferably sixth (seventh-period elements in the appropriate groups do not have widely known experimental chemistry). Again, earlier-period elements tend to be more oxophilic, especially in the later groups noted above.

  • Usually favoring chalcophilic or siderophilic elements in the Goldschmidt classification. Lithophile metals, which appear primarily in rocks rather than in ores or as native metals, combine preferentially with oxygen to make those rocks.

The case of antimony(V), a chalcophile element in G15 and P5, is summarized in atomistry.com. The acid and many salts are known, generally having yellow-green color which suggests that this is an intrinsic color of the anion. The +5 oxidation state of antimony is not strongly oxidizing enough to affect the chloride ligands. Most of the known salts are hydrated, especially with small cations, suggesting that the obtained acid is a hydronium salt like your examples.

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