The structure, the vibrational frequencies and the normal modes of C6ff are calculated with an upgraded QCFF/PI semiempirical quantum chemical method. We use a configuration interaction wavefunction expanded on 400 determinants (all the one-to six-electron excitations in the space spanned by the triply degenerate HOMO and the triply degenerate LUMO 0fC666 -are included). The optimized structure of the isolated hexaanion belongs to the IN point group. The two inequivalent bond lengths of the C6o frame are found to be similar. The frequency lowering observed upon doping is quantitatively ascribed to two mechanisms: the first is electronic in nature and is due to the bond order decrease caused by the addition of the extra six electrons to the antibonding orbitals of the neutral molecule; the second is of vibronic origin and is promoted by the mixing of electronic states caused by the vibrational motions. The normal modes of the hexaanion are projected onto the space of those of the neutral compound and the mixing caused by the additional charges is found to be negligible. The calculated frequencies are discussed in the light of recent infrared and Raman data of A6C6o (with A an alkali atom) and an inversion of the assignment of the la s (highest) and 2h s (second highest) frequencies is tentatively proposed. Analysis of the force field in internal coordinates finds that both the electronic and the vibronic mechanisms affect almost only the diagonal and the nearest neighbour stretch force constants.