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Multiple Bonding in Group 15

Multiple Bonding in Group 15. Like the group 14 compounds, catenation is not difficult with group 15. Reduction of the monochloride can lead to a single bond: 2 Me 2 ECl + 2Na  Me 2 E-EMe 2 + 2 NaCl This works best with bismuth and antimony, but is generalized over the entire group 15.

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Multiple Bonding in Group 15

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  1. Multiple Bonding in Group 15 Like the group 14 compounds, catenation is not difficult with group 15. Reduction of the monochloride can lead to a single bond: 2 Me2ECl + 2Na  Me2E-EMe2 + 2 NaCl This works best with bismuth and antimony, but is generalized over the entire group 15. In the case of arsenic, elimination of HCl is possible: Me2AsCl + Me2AsH  Me2As-AsMe2 + HCl

  2. Pyridine Analogues Analogues to pyridine for group 15 molecules have been around long enough to have two trivial names: Phosphabenzene, arsabenzene, stibabenzene, bismuthabenzene Phosphinine, arsinine, stibinine, bismuthinine They are made by methathesis with a tin alkyl. Substituted aryl rings are also known.

  3. Pyridine Analogues Because they have a different basicity than pyridine, these ring systems can be found as both s and h6 bonded ligands: Phosphorous and arsenic see the most employment as heteroatoms, due to their stronger bonds and greater participation in the aromatic system (which is weaker than nitrogen in pyridine). This trend in stability is rounded out by the Bi analogue being unstable.

  4. Pyrrole Analogues Pyrrole analogues with heavier members of group 15 are also known. These analogues can be dealkylated and form Cp-like anions. The trend in p system participation is the same as seen for the pyridine derivatives: decreasing down the group. These ligands, including pyrrole, have been shown to h5 coordinate to transition metal centres after the fashion of Cp.

  5. Multiple Bonding in Group 15 There were only two published structures of diarsenes Mes*AsAsCH(SiMe3)2 and [As{C(SiMe3)3}]2 by Cowley and co-workers. J. Am.Chem. Soc. 1983, 105, 5506. J. Chem. Soc., Dalton Trans. 1985, 383.

  6. A Bi=Bi unit Deep purple crystals Bi–Bi = 2.8206(8) Å, Bi–Bi–C = 100.5(2)° (Bi–Bi single bond = 2.990(2) Å in Ph2Bi–BiPh2) propose three orthogonal 6p orbitals without significant hybridization leads to a bond angle of approx. 90° Stable as a crystal even in air for several hours. Tokitoh, N.; Arai, Y.; Okazaki, R.; Nagase, S. Science 1997, 277, 78. Extended to include Sb analogues Sb=Sb 2.642(1) Å in:J. Am. Chem. Soc. 1998, 120, 433.

  7. Double Bonds in Group 15 Using large, protective ligands, Powers has made a series of compounds RECl2 and reduced them with magnesium to give single and double bonds: LiAr + ECl3 ArECl2 + LiCl J. Am. Chem. Soc. 1999, 121(14), 3363.

  8. Group 15 E=E Notably, the ligand either has Me (this example) or iPr groups on the ortho-position on the flanking rings: ArECl2 + Mg  ArE=EAr + MgCl2

  9. Group 15 E=E Dipnictenes are intensely colored - range from yellow or orange yellow to deep red-purple Two bands more intense p-p* and less intense n-p* The p-p* band shifts to longer wavelength as the group is descended - consistent with a weakening of the pbond

  10. Mechanism The mechanism isn’t clear:

  11. Mechanism (cont’d)

  12. Bonding Note that the angle at the pnictogen atom in dipnictenes narrows from ca.. 113.6(2)° in PhNNPh to 102.8(9)° in Mes*PPMes* and 100.5(2)° in Bi compounds. We expect the s-orbital character of the lone pairs also increases with increasing atomic number such that at Sb or Bi the lone pairs have mostly s-orbital character. Accordingly, the sand p bonds comprising the Sb=Sb and Bi=Bi double bonds are mostly composed from p-orbitals.

  13. Multiple Bonding in Group 15 Bi-Bi bond = 2.8769(5) Å in (CO)5W(BiR)2 (R = CH2SiMe3) Compare to Bi=Bi = 2.82-2.83 Å Organometallics 2003, 22, 2919.

  14. Multiple Bonding in Group 15 unprecedented opportunity to examine a dibismuthene “before” and “after” coordination Bi-Bi-C bond angles in 1 are 109.3(2) and 109.54(18)° - greater Bi-Bi bond distance in 1 is 3.1442(7) Å - lengthened Bi-Zr bond lengths (2.9903(10) and 3.0044(11) Å) may only be compared with 3.190 Å, the sum of the bismuth and zirconium covalent radii J. AM. CHEM. SOC. 2005, 127, 7672

  15. Multiple Bonding in Group 15 Bond options (1) Cp2Zr is a 14-electron fragment while (CO)5W is a 16- electron moiety (2) electron-rich Cp ligands tend to donate electron density to transition metals, whereas the strong p-acid (electron accepting) CO ligands decrease the electron density at the metal.

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