Pentamethylbismuth Explained
Pentamethylbismuth (or pentamethylbismuthorane[1]) is an organometalllic compound containing five methyl groups bound to a bismuth atom with formula Bi(CH3)5. It is an example of a hypervalent compound. The molecular shape is trigonal bipyramid.[2]
Production
Pentamethylbismuth is produced in a two step process. First, trimethylbismuth is reacted with sulfuryl chloride to yield dichloro trimethylbismuth, which is then reacted with two equivalents of methyllithium dissolved in ether.[3] The blue solution is cooled to −110 °C to precipitate the solid product.
Bi(CH3)3 + SO2Cl2 → Bi(CH3)3Cl2 + SO2
Bi(CH3)3Cl2 + 2LiCH3 → Bi(CH3)5 + 2LiCl
Properties
At -110 °C, Bi(CH3)5 is a blue-violet solid. The methyl groups are arranged in a trigonal bipyramid, and the bond-lengths of methyl with bismuth are all the same. However, the molecule is not rigid, as can be determined from the nuclear magnetic resonance spectrum that shows all methyl groups are equivalent.[3] It is stable as a solid, but in the gas phase, when heated or in solution decomposes to trimethylbismuth.[2] [3] The colour is unusual for bismuth or other hypervalent pnictide compounds, which are colourless. Calculations show that the colour is due to HOMO-LUMO transition. The HOMO is ligand based, whereas the LUMO is modified by relativistically stabilised bismuth 6s orbitals.[4]
Reactions
If excess methyllithium is used in production, an orange hexamethylbismuth salt, LiBi(CH3)6, is formed.[5] [3]
Extra reading
- Gagnon. Alexandre. Dansereau. Julien. Le Roch. Adrien. Organobismuth Reagents: Synthesis, Properties and Applications in Organic Synthesis. Synthesis. 2 March 2017. 49. 8. 1707–1745. 10.1055/s-0036-1589482. 102471505 .
Notes and References
- Hellwinkel. Dieter. Penta- and hexaorganyl derivatives of the main group five elements. Wittig Chemistry. Topics in Current Chemistry. 1883. 109. 1–63. 10.1007/BFb0018055. Topics in Current Chemistry. 3-540-11907-8.
- Greene. Tim M.. Downs. Anthony J.. Pulham. Colin R.. Haaland. Arne. Verne. Hans Peter. Volden. Hans Vidar. Timofeeva. Tatjana V.. Molecular Structures of Pentamethylarsenic(V) and Trimethyldichloroarsenic(V) by Gas Electron Diffraction and ab Initio Calculations:? Molecular Mechanics Calculations on Pentamethylarsenic(V), Pentaphenylarsenic(V), and Related Compounds. Organometallics. November 1998. 17. 24. 5287–5293. 10.1021/om980520r.
- Wallenhauer. Stephan. Seppelt. Konrad. Methylbismuth(V) Compounds. Angewandte Chemie International Edition in English. 18 May 1994. 33. 9. 976–978. 10.1002/anie.199409761.
- Conradie. Jeanet. Ghosh. Abhik. The Blue-Violet Color of Pentamethylbismuth: A Visible Spin-Orbit Effect. ChemistryOpen. February 2017. 6. 1. 15–17. 10.1002/open.201600131. 28168144. 5288750.
- Wallenhauer. Stephan. Seppelt. Konrad. Antimony(V) and Bismuth(V) Methyl Compounds: A Structural Comparison. Inorganic Chemistry. January 1995. 34. 1. 116–119. 10.1021/ic00105a021.
