  IBM-1} calculations for the fission products $^{108,110,112}$Ru have been
carried out. The even-even isotopes of Ru can be described as transitional
nuclei situated between the U(5) (spherical vibrator) and SO(6)
($\gamma$-unstable rotor) symmetries of the Interacting Boson Model. At first,
a Hamiltonian with only one- and two-body terms has been used. Excitation
energies and $B$(E2) ratios of gamma transitions have been calculated. A
satisfactory agreement has been obtained, with the exception of the odd-even
staggering in the quasi-$\gamma$ bands of $^{110,112}$Ru. The observed pattern
is rather similar to the one for a rigid triaxial rotor. A calculation based on
a Hamiltonian with three-body terms was able to remove this discrepancy. The
relation between the IBM and the triaxial rotor model was also examined.
