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What is the structure of the molecule $\ce{N5P3}$? $\ce{N5P3}$ is not documented online, so could anyone please comment on the structure of the molecule:

  • How many ligands does each phosphorous atom have?
  • How many ligands does each nitrogen atom have?
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I'm not sure why previous answer cited only the first paper on phosphorus(V) nitride $\ce{P3N5}$. A couple of years later the same team reported three polymorphs $\ce{α,β,γ-P3N5}$ with three-dimensional network. For $\ce{α-P3N5}$ and $\ce{γ-P3N5}$ crystal structures have been refined. Both modifications demonstrate vastly different structural types and coordination of pnictogens.

Low-pressure phase $\ce{α-P3N5}$

Data obtained via powder neutron diffraction with higher resolution by using synchrotron radiation. The crystal structure was determined by direct methods and refined by the Rietveld method. [1, 2]

Structure description from [1]:

In $\ce{α-P3N5}$ zweier single chains occur parallel to $[110]$ and $[1\bar{1}0]$, in which the $\ce{PN4}$ tetrahedra are connected alternately through edge- and corner-sharing. These zweier single chains are linked together through additional vertex-sharing $\ce{PN4}$ tetrahedra. [...] The network structure of connected $\ce{PN4}$ tetrahedra in $\ce{α-P3N5}$ could be characterized by analogy to condensed silicates by the specific distribution of the occurring $\ce{P_nN_n}$ ring sizes.

Liebau formula: $_\infty^3\ce{[P_3^{[4]} N_3^{[2]} N_2^{[3]}]}$ *

Crystal structure of α-P3N5

Figure 1. Crystal structure of $\ce{α-P3N5}$, view along $[010]$. Purple coordination tetrahedra correspond to $\ce{[PN4]}$.

Low-pressure phase $\ce{β-P3N5}$

$\ce{β-P3N5}$ was reported in a polymorphic mixture with $\ce{α-P3N5}$ and is it's stacking modification (no further crystallographic investigation has been published) [1]:

On the basis of the solved crystal structure of $\ce{α-P3N5}$, we are now working on the determination of structural models for $\ce{β-P3N5}$, and disordered $\ce{P3N5}$. The HRTEM results show that similar to the situation in the polytypes of the different stacking variants are based on the same structural modules, which are linked in different ways.

High-pressure phase $\ce{γ-P3N5}$

Data obtained via powder X-ray diffraction. The crystal structure of $\ce{γ-P3N5}$ was determined by direct methods and refined using the Rietveld method. [3]

Structure description from [3]:

The novel high-pressure modification consists of a polymeric three- dimensional network structure of linked $\ce{PN4}$ tetrahedra and tetragonal-pyramidal $\ce{PN5}$ units. [...] Rods from trans-edge-sharing $\ce{PN5}$ units running along $[010]$ are linked by vertices, leading to the formation of layers perpendicular to $[100]$. [...] In accordance with the pressure–coordination rule a partial increase of the coordination number of the phosphorus atoms from four in $\ce{α-P3N5}$ to five in $\ce{γ-P3N5}$ is observed. [...] The density of $\ce{γ-P3N5}$ is $32\%$ higher than the density of $\ce{α-P3N5}$. [...] Therefore the high-pressure phase could show interesting materials properties (e.g. great hardness or low compressibility).

Liebau formula: $\ce{P_5^{[5]}[ P^{[4]} N_3]N_2}$ *

Crystal structure of γ-P3N5

Figure 2. Crystal structure of $\ce{γ-P3N5}$, view along $[010]$. Purple coordination tetrahedra correspond to $\ce{[PN4]}$; yellow coordination pyramids — to $\ce{[PN5]}$.

* using Liebau's nomenclature for silicates, where the superscripted number in square brackets after the element symbol denotes the coordination number.

References

  1. Horstmann, S.; Irran, E.; Schnick, W. Synthesis and Crystal Structure of Phosphorus(V) Nitride α-P3N5. Angewandte Chemie International Edition in English 1997, 36 (17), 1873–1875. https://doi.org/10.1002/anie.199718731.
  2. Horstmann, S.; Irran, E.; Schnick, W. Phosphor(V)-nitrid $\ce{α-P3N5}$: Synthese ausgehend von Tetraaminophosphoniumiodid und Kristallstrukturaufklärung mittels Synchrotron-Pulver-Röntgenbeugung. Zeitschrift für anorganische und allgemeine Chemie 1998, 624 (4), 620–628. https://doi.org/10.1002/(SICI)1521-3749(199804)624:4<620::AID-ZAAC620>3.0.CO;2-K.
  3. Landskron, K.; Huppertz, H.; Senker, J.; Schnick, W. High-Pressure Synthesis of $\ce{γ-P3N5}$ at 11 GPa and 1500 °C in a Multianvil Assembly: A Binary Phosphorus(V) Nitride with a Three-Dimensional Network Structure from $\ce{PN4}$ Tetrahedra and Tetragonal $\ce{PN5}$ Pyramids. Angewandte Chemie International Edition 2001, 40 (14), 2643–2645. https://doi.org/10.1002/1521-3773(20010716)40:14<2643::AID-ANIE2643>3.0.CO;2-T.
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$\ce{N5P3}$ is more commonly written as $\ce{P3N5}$, and known as triphosphorus pentanitride. It's a crystalline solid at ambient conditions and not a molecular compound. From the first publication that reported the pure compound and its structure [1]:

In the solid a three-dimensional cross-linked network structure of corner sharing $\ce{PN4}$ tetrahedra has been identified with 2/5 of the nitrogen atoms bonded to three P atoms and 3/5 of the nitrogen atoms bonded to two P atoms.

enter image description here

Only the combination of spectroscopical and diffraction methods, especially electron microscopy for structural analysis, enables a detailed structural and crystallographic characterization of $\ce{P3N5}$ as no conventional single-crystal data for structure determination have been available for this compound.

A representative for an unique binary structure type was found for the first time which combines a tetrahedral structure with the very rare molar ratio of 3:5 for the constituent elements.

References

  1. Schnick, W.; Lücke, J.; Krumeich, F. Chem. Mater. 1996, 8 (1), 281–286 DOI: 10.1021/cm950385y.
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