Pentamethylbismuth (or pentamethylbismuthorane[2]) is an organometalllic compound containing five methyl groups bound to a bismuth atom with formula Bi(CH3)5.

However, the molecule is not rigid, as can be determined from the nuclear magnetic resonance spectrum that shows all methyl groups are equivalent.

[4] It is stable as a solid, but in the gas phase, when heated or in solution decomposes to trimethylbismuth.

The HOMO is ligand based, whereas the LUMO is modified by relativistically stabilised bismuth 6s orbitals.

[5] If excess methyllithium is used in production, an orange hexamethylbismuth salt, LiBi(CH3)6, is formed.