Pentamethylarsenic (or pentamethylarsorane[1]) is an organometalllic compound containing five methyl groups bound to an arsenic atom with formula As(CH3)5.

[2] The first claim to make pentamethylarsenic was in 1862 in a reaction of tetramethylarsonium iodide with dimethylzinc by A.

[8] The infrared spectrum of pentamethylarsenic shows strong bands at 582 and 358 cm−1 due to axial C-As vibration, and weaker bands at 265 and 297 cm−1 due to equatorial C-As vibration.

[1] When pentamethylarsenic is heated to 100° it decomposes forming trimethylarsine, methane, and ethylene.

[10] When trimethylindium reacts with pentamethylarsenic in benzene solution, a salt precipitates: tetramethylarsenic(V)tetramethylindate(III).