Pentamethylbismuth (or pentamethylbismuthorane) is an organometalllic compound containing five

methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...

s bound to a

bismuth Bismuth is a chemical element; it has symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs nat ...

atom with formula Bi(CH₃)₅. It is an example of a

hypervalent In chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded Octet rule, octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. P ...

compound. The molecular shape is

trigonal bipyramid A triangular bipyramid is a hexahedron with six triangular faces constructed by attaching two tetrahedra face-to-face. The same shape is also known as a triangular dipyramid or trigonal bipyramid. If these tetrahedra are regular, all faces of a t ...

Production

Pentamethylbismuth is produced in a two step process. First, trimethylbismuth is reacted with

sulfuryl chloride Sulfuryl chloride is an inorganic compound with the formula SO2Cl2. At room temperature, it is a colorless liquid with a pungent odor. Sulfuryl chloride is not found in nature. Sulfuryl chloride is commonly confused with thionyl chloride, SOC ...

to yield dichloro trimethylbismuth, which is then reacted with two equivalents of

methyllithium Methyllithium is the simplest organolithium reagent, with the empirical formula LiCH3. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used i ...

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 Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

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 , - ! colspan=2 style="text-align:left;" , ↓ Period , - ! 2 , , - ! 3 , , - ! 4 , , - ! 5 , , - ! 6 , , - ! 7 , , - , colspan="2", ---- ''Legend'' A pnictogen ( or ; from "to choke" and -gen, "generator") is any ...

compounds, which are colourless. Calculations show that the colour is due to

HOMO ''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...

-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.

References

Extra reading

* {{bismuth compounds Organobismuth compounds Hypervalent molecules Methyl complexes