We present relativistic calculations at the four-component Dirac-DFT level for the geometries of the series of group 9 monoanionic hexafluorides MF6-, M = Co, Rh, Ir. Highly correlated four-component relativistic CCSD(T) energies were also calculated for the optimized geometries. Spin-orbit coupling effects influence the geometrical preferences for molecular structures: relativistic calculations predict ground states with octahedral symmetries O-h* for all hexafluorides in this study, while at the nonrelativistic limit, a structural deviation toward Do ground state symmetries is predicted. Our findings suggest that relativistic effects have an important role in molecular structure preferences for the title hexafluorides.