We measured the permanent electric dipole moment of transition metal-(C6H6)(2) sandwich complexes in the gas phase. The electric dipole for complexes with Sc, Ti, V, Nb, Ta, and Zr is zero (mu < 0.2 D), which is in agreement with a symmetric structure. The electric dipole on the A principal inertial axis of the molecule is 0.7 +/- 0.3 D for Co-(C6H6)(2), and 1.3 +/- 0.3 D for Ni-(C6H6)(2). This shows that they have an asymmetric structure. (C) 2003 Elsevier Science B.V. All rights reserved.