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What are the chains and cages of phosphorus, nitrogen, and sulphur?
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Lucas Curtis
What are the chains and cages of phosphorus, nitrogen, and sulphur?
Although the chemistry of few nonmetallic elements is characterised by their capabilities to form many bonds with themselves - much like carbon or boron do, that is, the formation of chains or cages structures for phosphorus, nitrogen and sulphur is less important than those of Group 14.
Nonetheless there exist some remarkable features from these points of view.
Chains.
They are termed as homochains if they are made up of the same element or heterochains if some other element comes into play.
In the case of phosphorus homochains, there are few compounds with P-P atom sequences that can be considered as worth mentioning: if my memory servers, they are diphosphine (P₂H₆), triphosphine (H₂PPHPH₂) and tetraphosphine (H₂PPHPHPH₂). Such open chain structures seem to be inherently unstable with respect to the analogous cyclic ones.
In the case of nitrogen, although chain lengths of up to 7–8 atoms are documented, there are only a handful of compounds that are truly stable at room temperature: off the top of my mind, hydrazine (H₂N-NH₂), hydrazoic acid (HN₃), and 2-tetrazene (H₂N-N=N-NH₂. The bulkier the substituents instead of H the more stable 2-tetrazene becomes.
In the case of sulphur, the so-called sulphanes H₂S? are well known, wherein ? goes up to eight. However, the only sulphane that turns out to be thermodynamically stable is however the stinky (and poisonous) H₂S.
On the other hand, heterocatenation seems to be quite widespread: a different (“hetero”) atom gets in between these nonmetals to enhance stability. Typical examples of this behaviour are
silicones -(R₂SiO)?-
polyphosphazenes -[PN(OR)₂]?-
In both cases, oxygen atoms are intercalated to ensure stability to the chains. For sulphur compounds, the only one that has some importance is pyrosulphuric acid, H₂S₂O₇.
An unusual hetero chain, from this point of view, is polythiazyl (SN)?: discovered in early 20th century, only in the seventies was it found out to be an electrical conductor. X-ray studies showed that polythiazyl chains can be thought to arise from square planar S₂N₂ molecules.
Other sets of interesting structures with P, N and other atoms (mainly B and halogens) are the so-called borazines and cyclophosphazene compounds:
borazines have the general formula B₃N₃R₆ (R = H, alkyl);
cyclophosphazenes have the general formula P₃N₃X₆ (X = F, Cl, Br).
Both cyclic structures can be thought to be the inorganic analogues of benzene C₆H₆. Some remarkable differences are at play, however: borazine’s ?-electron cloud displays uneven distribution, for instance. Moreover, by varying the nature of the R side chains of cyclophosphazenes, one can come up with useful materials (fibres, elastomers, plastics) that are water and fire resistant. Some fluoro derivatives seem to be so water repellent that they may have promising applications in prosthetic devices.
Some interesting chains of S are the so-called polysulfides: S₂²¯, S₃²¯, S₄²¯, S₅²¯. They all have open chain structures. Cyclic phosphines (RP)?, with ? = 3 to 6 and R = alkyl group, are known, too.
Cages.
Apart from allotropic structures of elemental phosphorus and sulphur, P₄ and S₈ (no allotropes are known for nitrogen), homocyclic rings are known for the latter with different numbers of atoms (six to thirty-six). They are not thermodynamically stable.
Interesting mixed compounds with cage-like structures can be synthesised by reacting phosphorus and sulphur together: the most known ones are
Although the chemistry of few nonmetallic elements is characterised by their capabilities to form many bonds with themselves - much like carbon or boron do, that is, the formation of chains or cages structures for phosphorus, nitrogen and sulphur is less important than those of Group 14.
Nonetheless there exist some remarkable features from these points of view.
Chains.
They are termed as homochains if they are made up of the same element or heterochains if some other element comes into play.
In the case of phosphorus homochains, there are few compounds with P-P atom sequences that can be considered as worth mentioning: if my memory servers, they are diphosphine (P₂H₆), triphosphine (H₂PPHPH₂) and tetraphosphine (H₂PPHPHPH₂). Such open chain structures seem to be inherently unstable with respect to the analogous cyclic ones.
In the case of nitrogen, although chain lengths of up to 7–8 atoms are documented, there are only a handful of compounds that are truly stable at room temperature: off the top of my mind, hydrazine (H₂N-NH₂), hydrazoic acid (HN₃), and 2-tetrazene (H₂N-N=N-NH₂. The bulkier the substituents instead of H the more stable 2-tetrazene becomes.
In the case of sulphur, the so-called sulphanes H₂S? are well known, wherein ? goes up to eight. However, the only sulphane that turns out to be thermodynamically stable is however the stinky (and poisonous) H₂S.
On the other hand, heterocatenation seems to be quite widespread: a different (“hetero”) atom gets in between these nonmetals to enhance stability. Typical examples of this behaviour are
silicones -(R₂SiO)?-
polyphosphazenes -[PN(OR)₂]?-
In both cases, oxygen atoms are intercalated to ensure stability to the chains. For sulphur compounds, the only one that has some importance is pyrosulphuric acid, H₂S₂O₇.
An unusual hetero chain, from this point of view, is polythiazyl (SN)?: discovered in early 20th century, only in the seventies was it found out to be an electrical conductor. X-ray studies showed that polythiazyl chains can be thought to arise from square planar S₂N₂ molecules.
Other sets of interesting structures with P, N and other atoms (mainly B and halogens) are the so-called borazines and cyclophosphazene compounds:
borazines have the general formula B₃N₃R₆ (R = H, alkyl);
cyclophosphazenes have the general formula P₃N₃X₆ (X = F, Cl, Br).
Both cyclic structures can be thought to be the inorganic analogues of benzene C₆H₆. Some remarkable differences are at play, however: borazine’s ?-electron cloud displays uneven distribution, for instance. Moreover, by varying the nature of the R side chains of cyclophosphazenes, one can come up with useful materials (fibres, elastomers, plastics) that are water and fire resistant. Some fluoro derivatives seem to be so water repellent that they may have promising applications in prosthetic devices.
Some interesting chains of S are the so-called polysulfides: S₂²¯, S₃²¯, S₄²¯, S₅²¯. They all have open chain structures. Cyclic phosphines (RP)?, with ? = 3 to 6 and R = alkyl group, are known, too.
Cages.
Apart from allotropic structures of elemental phosphorus and sulphur, P₄ and S₈ (no allotropes are known for nitrogen), homocyclic rings are known for the latter with different numbers of atoms (six to thirty-six). They are not thermodynamically stable.
Interesting mixed compounds with cage-like structures can be synthesised by reacting phosphorus and sulphur together: the most known ones are
Although the chemistry of few nonmetallic elements is characterised by their capabilities to form many bonds with themselves - much like carbon or boron do, that is, the formation of chains or cages structures for phosphorus, nitrogen and sulphur is less important than those of Group 14.
Nonetheless there exist some remarkable features from these points of view.
Chains.
They are termed as homochains if they are made up of the same element or heterochains if some other element comes into play.
In the case of phosphorus homochains, there are few compounds with P-P atom sequences that can be considered as worth mentioning: if my memory servers, they are diphosphine (P₂H₆), triphosphine (H₂PPHPH₂) and tetraphosphine (H₂PPHPHPH₂). Such open chain structures seem to be inherently unstable with respect to the analogous cyclic ones.
In the case of nitrogen, although chain lengths of up to 7–8 atoms are documented, there are only a handful of compounds that are truly stable at room temperature: off the top of my mind, hydrazine (H₂N-NH₂), hydrazoic acid (HN₃), and 2-tetrazene (H₂N-N=N-NH₂. The bulkier the substituents instead of H the more stable 2-tetrazene becomes.
In the case of sulphur, the so-called sulphanes H₂S? are well known, wherein ? goes up to eight. However, the only sulphane that turns out to be thermodynamically stable is however the stinky (and poisonous) H₂S.
On the other hand, heterocatenation seems to be quite widespread: a different (“hetero”) atom gets in between these nonmetals to enhance stability. Typical examples of this behaviour are
In both cases, oxygen atoms are intercalated to ensure stability to the chains. For sulphur compounds, the only one that has some importance is pyrosulphuric acid, H₂S₂O₇.
An unusual hetero chain, from this point of view, is polythiazyl (SN)?: discovered in early 20th century, only in the seventies was it found out to be an electrical conductor. X-ray studies showed that polythiazyl chains can be thought to arise from square planar S₂N₂ molecules.
Other sets of interesting structures with P, N and other atoms (mainly B and halogens) are the so-called borazines and cyclophosphazene compounds:
Both cyclic structures can be thought to be the inorganic analogues of benzene C₆H₆. Some remarkable differences are at play, however: borazine’s ?-electron cloud displays uneven distribution, for instance. Moreover, by varying the nature of the R side chains of cyclophosphazenes, one can come up with useful materials (fibres, elastomers, plastics) that are water and fire resistant. Some fluoro derivatives seem to be so water repellent that they may have promising applications in prosthetic devices.
Some interesting chains of S are the so-called polysulfides: S₂²¯, S₃²¯, S₄²¯, S₅²¯. They all have open chain structures. Cyclic phosphines (RP)?, with ? = 3 to 6 and R = alkyl group, are known, too.
Cages.
Apart from allotropic structures of elemental phosphorus and sulphur, P₄ and S₈ (no allotropes are known for nitrogen), homocyclic rings are known for the latter with different numbers of atoms (six to thirty-six). They are not thermodynamically stable.
Interesting mixed compounds with cage-like structures can be synthesised by reacting phosphorus and sulphur together: the most known ones are
Fascinating chemistry.
Although the chemistry of few nonmetallic elements is characterised by their capabilities to form many bonds with themselves - much like carbon or boron do, that is, the formation of chains or cages structures for phosphorus, nitrogen and sulphur is less important than those of Group 14.
Nonetheless there exist some remarkable features from these points of view.
Chains.
They are termed as homochains if they are made up of the same element or heterochains if some other element comes into play.
In the case of phosphorus homochains, there are few compounds with P-P atom sequences that can be considered as worth mentioning: if my memory servers, they are diphosphine (P₂H₆), triphosphine (H₂PPHPH₂) and tetraphosphine (H₂PPHPHPH₂). Such open chain structures seem to be inherently unstable with respect to the analogous cyclic ones.
In the case of nitrogen, although chain lengths of up to 7–8 atoms are documented, there are only a handful of compounds that are truly stable at room temperature: off the top of my mind, hydrazine (H₂N-NH₂), hydrazoic acid (HN₃), and 2-tetrazene (H₂N-N=N-NH₂. The bulkier the substituents instead of H the more stable 2-tetrazene becomes.
In the case of sulphur, the so-called sulphanes H₂S? are well known, wherein ? goes up to eight. However, the only sulphane that turns out to be thermodynamically stable is however the stinky (and poisonous) H₂S.
On the other hand, heterocatenation seems to be quite widespread: a different (“hetero”) atom gets in between these nonmetals to enhance stability. Typical examples of this behaviour are
In both cases, oxygen atoms are intercalated to ensure stability to the chains. For sulphur compounds, the only one that has some importance is pyrosulphuric acid, H₂S₂O₇.
An unusual hetero chain, from this point of view, is polythiazyl (SN)?: discovered in early 20th century, only in the seventies was it found out to be an electrical conductor. X-ray studies showed that polythiazyl chains can be thought to arise from square planar S₂N₂ molecules.
Other sets of interesting structures with P, N and other atoms (mainly B and halogens) are the so-called borazines and cyclophosphazene compounds:
Both cyclic structures can be thought to be the inorganic analogues of benzene C₆H₆. Some remarkable differences are at play, however: borazine’s ?-electron cloud displays uneven distribution, for instance. Moreover, by varying the nature of the R side chains of cyclophosphazenes, one can come up with useful materials (fibres, elastomers, plastics) that are water and fire resistant. Some fluoro derivatives seem to be so water repellent that they may have promising applications in prosthetic devices.
Some interesting chains of S are the so-called polysulfides: S₂²¯, S₃²¯, S₄²¯, S₅²¯. They all have open chain structures. Cyclic phosphines (RP)?, with ? = 3 to 6 and R = alkyl group, are known, too.
Cages.
Apart from allotropic structures of elemental phosphorus and sulphur, P₄ and S₈ (no allotropes are known for nitrogen), homocyclic rings are known for the latter with different numbers of atoms (six to thirty-six). They are not thermodynamically stable.
Interesting mixed compounds with cage-like structures can be synthesised by reacting phosphorus and sulphur together: the most known ones are
Fascinating chemistry.
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