The kinetic and electronic stability of a series of free LnF6q-type lanthanide hexafluorides (Ln = Ce to Lu; q = 2, 3) is studied with the relativistic effective core potentials of Cundari and Stevens. In all complexes (LnF62− and LnF63−), the octahedral or almost octahedral structure is calculated to be stable through normal mode analyses at the unrestricted Hartree–Fock (UHF) and complete active space self-consistent field (CASSCF) levels. The electronic stability of all complexes is investigated using the state-averaged CASSCF and quasidegenerate perturbation theory with multiconfigurational self-consistent field reference functions (MC-QDPT) schemes. The most electronically stable LnF6q is (LnF62− for Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho, Er, Tm, Yb. These LnF62− anions are found to have an energy barrier with respect to the unimolecular decomposition LnF62− → LnF5 + F and are predicted to be long-lived species formally existing in a meta-stable state.