33654_12a-1 - Previous Page 482 Ch 12 Phosphorus Figure...

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482 Phosphorus Ch. 12 Figure 12.4 Structure of Hittorf's violet monoclinic phosphorus showing (a) end view of one pentagonal tube, (b) the side view of a single tube (dimensions in pm). same as in P4) but the average P-P-P angle is 101" (instead of 60"). Black phosphorus, the thermodynamically most stable form of the element, has been prepared in three crystalline forms and one amorphous form. It is even more highly polymeric than the red form and has a correspondingly higher density (orthorhombic 2.69, rhombohedral 3.56, cubic 3.88 g ~m-~). Black phosphorus (orthorhombic) was originally made by heating white P4 to 200" under a pressure of 12000 atm (P. W. Bridgman, 1916). Higher pressures convert it successively to the rhombohedral and cubic forms (Fig. 12.3). Orthorhombic black P (mp -610") has a layer structure which is based on a puckered hexagonal net of 3-coordinate P atoms with 2 interatomic angles of 102" and 1 of 96.5" (P-P 223pm). The relation of this form to the rhombohedral and cubic forms is shown in Fig. 12.5. Comparison with the rhombohedral forms of As, Sb and Bi is also instructive in showing the increasing tendency towards octahedral coordination and metallic properties (p. 551). Black P is semiconducting but its electrical properties are probably significantly affected by impurities introduced during its preparation. 12.2.4 Atomic and physical properties g6) Phosphorus has only one stable isotope, ;iP, and accordingly (p. 17) its atomic weight is known with extreme accuracy, 30.973 762(4). Sixteen radioactive isotopes are known, of which 32P is by far the most important; it is made on the multikilogram scale by the neutron irradiation of 32S(n,p) or 31P(n,y) in a nuclear reactor, and is a pure p-emitter of half life 14.26 days, E,,, 1.709 MeV, E,,,, 0.69 MeV. It finds extensive use in tracer and mechanistic studies. The stable isotope 31P has a nuclear spin quantum number of and this is much used in nmr spectroscopy.(27) Chemical shifts and coupling constants can both be used diagnostically to determine structural information. In the ground state, P has the electronic configuration [Ne]3s23p13p:3pf with 3 unpaired 26 Mellor 's Comprehensive Treatise on Inorganic and The- oretical Chemistry, Vol. 8, Suppl. 3, Phosphorus, Longman, London, 1971, 1467 pp. 21 D. G. GORENSTEIN (ed.) Phosphorus-31 NMR; Principles and Applications Academic Press, London, 1984, 604 pp. J. G. VERKADE and L. D. QUIN (eds.), Phosphorus-31 NMR Spectroscopy in Stereochemical Analysis, VCH Publishers, Weinheim, 1987, 717 pp. Previous Page
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812.2.5 Chemical reactivity and stereochemistry 483 Figure 12.5 The structures of black phosphorus: (a) portion of one layer of orthorhombic P (idealized), (b) rhombohedral form, portion of one hexagonal layer, (c) cubic form, 4 unit cells, (d) distortion of (a) to the cubic form, and (e) distortion of (b) to the cubic form. electrons; this, together with the availability of low-lying vacant 3d orbitals, accounts for the pre- dominant oxidation states I11 and V in phosphorus chemistry. Ionization energies, electronegativity, and atomic radii are compared with those of N, As, Sb and Bi on p.
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33654_12a-1 - Previous Page 482 Ch 12 Phosphorus Figure...

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