Molecular CsF 5 and CsF 2 +

D 5 h star‐like CsF 5 , formally isoelectronic with known XeF 5 − ion, is computed to be a local minimum on the potential energy surface of CsF 5 , surrounded by reasonably large activation energies for its exothermic decomposition to CsF+2 F 2 , or to CsF 3  (three isomeric forms)+F 2 , or for rearrangement to a significantly more stable isomer, a classical Cs + complex of F 5 − . Similarly the CsF 2 + ion is computed to be metastable in two isomeric forms. In the more symmetrical structures of these molecules there is definite involvement in bonding of the formally core 5p levels of Cs.