The chlorination of the carboxylic acid unit of α-aminoacids is an important reaction in organic synthesis, and the resulting compounds are usually seen as reactive intermediate species. Herein we report a straightforward procedure to obtain [MCl₆]⁻ salts (M = Nb, 2a; Ta, 2b) of the L-proline-derivative (pyrrolidinium-2-carbonylchloride). These are stable up to 80 °C, in the solid state as well as in organic solvents. The X-ray structures determined for 2a,b include the first example of a crystallographically-characterized α-ammonium-acylchloride.