In the reaction of [B 6 H 6 ] 2 - with benzyl bromide in dichloromethane the protonated anion [B 6 H 5 H f a c CH 2 Ph] - is formed. The crystal structure of (Ph 4 P)[B 6 H 5 H f a c CH 2 Ph] has been determined by single crystal diffraction analysis: monoclinic, space group P2 1 /c with a=13.795(5), b=11.133(5), c=19.231(5) 9, β=110 659(5) o , Z=4, R=0.056. Typical distances of 1.59, 1.73 and 1.13 9 are averaged out for B-C, B-B and B-H, respectively. One of the four facets, which shares the ipso boron atom, is significantly expanded suggesting that the additional proton H f a c is fixed above this facet by a 2e4c bond in the solid state. In solution and at room temperature the proton is fluctuating across the four ipso facets as shown by variable temperature 1 1 B NMR spectroscopy, and the point of coalescence for its fixation is found at 234 K in dichloromethane solution in a field of 9.4 T. A second point of coalescence according to the fixation of the benzyl group is observed at 225 K. Additionally, a nuclear Overhauser interaction between H f a c and the alkyl H atoms has been proven by 1 H- 1 H NOESY below the first coalescence temperature. All data from vibrational and 1 H, 1 1 B, 1 3 C NMR spectroscopy are consistent with the structural features.