Thallium bis(trimethylsilyl)amide is a monomer in the gas phase. The molecular symmetry is C(2), the Tl-N bond length is 214.8(12) pm by gas electron diffraction as compared to about 258 pm in the crystalline dimer (Klinkhammer, K. W.; Henkel, S. J. Organomet. Chem. 1994, 480, 167). Structure optimization by DFT or ab initio (MP2) calculations with the ECP basis set on the Tl atom fails to reproduce the experimental Tl-N bond distance unless the 5s,5p,5d electrons on the metal are described explicitly and the atomic basis set includes both polarization and diffuse functions.