Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my knowledge) hydrogen phosphonates rather than dihydrogen phosphites. Similarly, all $\ce{HPO3^2-}$ salts should be phophonates rather than hydrogen phosphate. Only as organic esters (which cannot tautomerise easily as carbocations are less stable) are both forms observed: trimethyl phosphite and dimethyl methylphosphonate both exist.

A similar picture is true for sulfur in the $\mathrm{+IV}$ state. Sulfurous acid and sulphonic acid are both known as far as I know, and both structures have been isolated in salts. Similarly, methyl methylsulfonate and dimethyl sulfite are both known to the best of my knowledge.

The corresponding elements of the second period do not show this tautomerism. For oxygen, this is due to its high negativity preventing the formation of ‘oxyacids’ altogether. For nitrogen, this is because an $\ce{N=O}$ double bond is more stable than two $\ce{N-O}$ single bonds so that nitrite ($\ce{NO2-}$) is already devoid of protons and cannot tautomerise. However, remnants of this behaviour still exists in organic compounds where both the nitro group $\ce{-NO2}$ and organic nitrites $\ce{-ONO}$ are known.

What is the case for the heavier homologues of both phosphorus and sulfur? Do the elements arsenic/antimony/bismuth and selenium/tellurium/polonium exhibit both an -onic acid and an -ous acid or is one form preferred over the other? Is there a general trend going down the groups or are secondary effects (e.g. lanthanide contraction) more important?

Answers backed with references to crystal structures of anions (or the free acids) and/or corresponding quantum-mechanical calculations (both in references or self-performed) are preferred.

Note that the fully oxidised pnictogen(V) or chalcogen(VI) compounds are not part of this question,

Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my knowledge) hydrogen phosphonates rather than dihydrogen phosphites. Similarly, all $\ce{HPO3^2-}$ salts should be phophonates rather than hydrogen phosphate. Only as organic esters (which cannot tautomerise easily as carbocations are less stable) are both forms observed: trimethyl phosphite and dimethyl methylphosphonate both exist.

A similar picture is true for sulfur in the $\mathrm{+IV}$ state. Sulfurous acid and sulphonic acid are both known as far as I know, and both structures have been isolated in salts. Similarly, methyl methylsulfonate and dimethyl sulfite are both known to the best of my knowledge.

The corresponding elements of the second period do not show this tautomerism. For oxygen, this is due to its high negativity preventing the formation of ‘oxyacids’ altogether. For nitrogen, this is because an $\ce{N=O}$ double bond is more stable than two $\ce{N-O}$ single bonds so that nitrite ($\ce{NO2-}$) is already devoid of protons and cannot tautomerise. However, remnants of this behaviour still exists in organic compounds where both the nitro group $\ce{-NO2}$ and organic nitrites $\ce{-ONO}$ are known.

What is the case for the heavier homologues of both phosphorus and sulfur? Do the elements arsenic/antimony/bismuth and selenium/tellurium/polonium exhibit both an -onic acid and an -ous acid or is one form preferred over the other? Is there a general trend going down the groups or are secondary effects (e.g. lanthanide contraction) more important?

Answers backed with references to crystal structures of anions (or the free acids) and/or corresponding quantum-mechanical calculations (both in references or self-performed) are preferred.

Note that the fully oxidised pnictogen(V) or chalcogen(VI) compounds are not part of this question,

Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my knowledge) hydrogen phosphonates rather than dihydrogen phosphites. Similarly, all $\ce{HPO3^2-}$ salts should be phophonates rather than hydrogen phosphate. Only as organic esters (which cannot tautomerise easily as carbocations are less stable) are both forms observed: trimethyl phosphite and dimethyl methylphosphonate both exist.

A similar picture is true for sulphur in the $\mathrm{+IV}$ state. Sulphurous acid and sulphonic acid are both known as far as I know, and both structures have been isolated in salts. Similarly, methyl methylsulphonate and dimethyl sulphite are both known to the best of my knowledge.

The corresponding elements of the second period do not show this tautomerism. For oxygen this is due to its high negativity preventing the formation of ‘oxyacids’ altogether. For nitrogen, this is because an $\ce{N=O}$ double bond is more stable than two $\ce{N-O}$ single bonds, so that nitrite ($\ce{NO2-}$) is already ridded of protons and cannot tautomerise. However, remnants of this behaviour still exist in organic compounds where both the nitro group $\ce{-NO2}$ and organic nitrites $\ce{-ONO}$ are known.

What is the case for the heavier homologues of both phosphorus and sulphur? Do the elements arsenic/antimony/bismuth and selenium/tellurium/polonium exhibit both an -onic acid and an -ous acid or is one form preferred over the other? Is there a general trend going down the groups or are secondary effects (e.g. lanthanide contraction) more important?

Answers backed with references to crystal structures of anions (or the free acids) and/or corresponding quantum-mechanical calculations (both in references or self-performed) are preferred.

Note that the fully oxidised pnictogen(V) or chalcogen(VI) compounds are not part of this question,

Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my knowledge) hydrogen phosphonates rather than dihydrogen phosphites. Similarly, all $\ce{HPO3^2-}$ salts should be phophonates rather than hydrogen phosphate. Only as organic esters (which cannot tautomerise easily as carbocations are less stable) are both forms observed: trimethyl phosphite and dimethyl methylphosphonate both exist.

A similar picture is true for sulfur in the $\mathrm{+IV}$ state. Sulfurous acid and sulphonic acid are both known as far as I know, and both structures have been isolated in salts. Similarly, methyl methylsulfonate and dimethyl sulfite are both known to the best of my knowledge.

The corresponding elements of the second period do not show this tautomerism. For oxygen, this is due to its high negativity preventing the formation of ‘oxyacids’ altogether. For nitrogen, this is because an $\ce{N=O}$ double bond is more stable than two $\ce{N-O}$ single bonds so that nitrite ($\ce{NO2-}$) is already devoid of protons and cannot tautomerise. However, remnants of this behaviour still exists in organic compounds where both the nitro group $\ce{-NO2}$ and organic nitrites $\ce{-ONO}$ are known.

What is the case for the heavier homologues of both phosphorus and sulfur? Do the elements arsenic/antimony/bismuth and selenium/tellurium/polonium exhibit both an -onic acid and an -ous acid or is one form preferred over the other? Is there a general trend going down the groups or are secondary effects (e.g. lanthanide contraction) more important?

Answers backed with references to crystal structures of anions (or the free acids) and/or corresponding quantum-mechanical calculations (both in references or self-performed) are preferred.

Note that the fully oxidised pnictogen(V) or chalcogen(VI) compounds are not part of this question,