Pentamethylbismuth Energy States on:
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Pentamethylbismuth (or pentamethylbismuthorane) 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
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. It is stable as a solid, but in the gas phase, when heated or in solution decomposes to trimethylbismuth.
The colour is unusual for bismuth or other hypervalent
trigonal bipyramid
In geometry, the triangular bipyramid (or dipyramid) is a type of hexahedron, being the first in the infinite set of face-transitive bipyramids. It is the dual of the triangular prism with 6 isosceles triangle faces.
As the name suggests, i ...
.
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 ofmethyllithium
Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in so ...
dissolved in ether. 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. It is stable as a solid, but in the gas phase, when heated or in solution decomposes to trimethylbismuth.
The colour is unusual for bismuth or other hypervalent pnictide
A pnictogen ( or ; from grc, πνῑ́γω "to choke" and -gen, "generator") is any of the chemical elements in group 15 of the periodic table. Group 15 is also known as the nitrogen group or nitrogen family. Group 15 consists of the el ...
compounds, which are colourless. Calculations show that the colour is due to HOMO
''Homo'' () is the genus that emerged in the (otherwise extinct) genus ''Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely relate ...
-LUMO transition. The HOMO is ligand based, whereas the LUMO is modified by relativistically stabilised bismuth 6s orbitals.
Reactions
If excess methyllithium is used in production, an orange hexamethylbismuth salt, LiBi(CH3)6, is formed . This is unusual as very few elements can form bonds with six organic groups.References
Extra reading
* {{open access Organobismuth compounds Hypervalent molecules Methyl complexes