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The Structure determination of some cyclic phosphonitrilic compounds Whitlow, Simon H.

Abstract

The structures of four inorganic compounds have been determined by single-crystal X-ray diffraction methods. Three of the structures investigated were phosphonitrilic derivatives: octamethylcyclotetra-phosphonitrilium trichlorocopper(II), [NPMe₂]₄H.CuCl₃, bis-(octa-methylcyclotetraphosphonitrilium) tetrachlorocobaltate(II), [(NPMe₂)₄H⁺] ₂ CoCl₄²ˉ , and hexadecamethoxycyclo-octaphosphonitrile, [NP(OMe)₂]₈. The fourth analysis was a study of the ionic compound caesium difluorophosphate. For each structure determination the intensity measurements were collected on a diffractometer using a scintillation counter and Mo-K̲⍺ radiation. The structure of [NPMe₂]₄H.CuCl₃ was determined from Patterson and Fourier summations. Refinement of positional and thermal parameters of the atoms was by least squares. The structure consists of discrete molecules in which the eight-member phosphonitrilic ring approximates the "tub" conformation with pairs of adjacent phosphorus and nitrogen atoms displaced alternately above and below the ring plane. A ring nitrogen atom is bonded to the copper atom (N-Cu = 2.04 Å) which has three chlorines arranged about it in a distorted square planar configuration. Across the ring from the N-Cu bond, there is a protonated nitrogen atom which is hydrogen-bonded to a neighbouring chlorine. The phosphorus-nitrogen bond lengths are not equal around the ring, but occur instead in four distinct pairs having lengths 1.63 Å, 1.60 Å, 1.56 Å, and 1.67 Å. These varying bond distances are explainable in terms of π-bonding systems. The structure of [ (NPMe₂)₄H⁺] ₂CoCl₄²ˉ has been determined by Patterson and Fourier methods and refined by least-squares. Most of the methyl hydrogen atoms were located in a final difference Fourier summation. The structure consists of tetrahedral CoCl₄²ˉ ions hydrogen-bonded to two protonated phosphonitrilic rings. The N-H...C1 distance in each case is 3.21 Å. The two independent rings have slightly different conformations. One approaches the "tub" shape and the other tends towards the "saddle" conformation. Protonation of the phosphonitrilic rings again results in non-equivalent P-N bond lengths. Averaged values for the two rings are 1.69 Å, 1.54 Å, 1.61 Å and 1.58 Å. Parameters which are unaffected by the addition of the proton agree with those found in related compounds when averaged values are considered. [NP(OMe)₂]₈ is a centrosymmetric sixteen-member phosphonitrile. Its structure was determined by Patterson, electron density, and least squares techniques. The phosphonitrilic ring consists of two approximately planar and parallel six-atom segments joined by a step. The P-N bond lengths are all equal within experimental error, their mean being 1.561 Å. The average P-0 and 0-C bond distances are 1.576 Å and 1.440 Å respectively. The methoxy groups cover the ring evenly, their arrangement being such that there is little conflict between steric and π- bonding requirements. The structure of caesium difluorophosphate was determined by comparison with KPO₂F₂ and refined by least square methods. Caesium and rubidium difluorophosphate are isomorphous with the potassium salt; all have the barium sulfate structure. The mean P-F distance in CsP0₂F₂ is 1.58 Å, the P-0 bond length, 1.48 Å. The lattice parameters and interionic distances increase with increasing radius of the alkali-metal cation.

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