The geometrical structure of CF 5 − has been ab initio investigated, taking all 52 electrons into account, using the MOLPRO-Programm/SCF-MO-LC-(LCGO)-Method. After approximating the core by means of the energy variation method and varying the η p -values and the C - F-distance a total energy of - 517,7008 a. u. with a C - F 4,5 -distance of 2.78 a. u. and a C - F 1,2,3 -distance of 2.74 a. u. for the CF 5 − -system of the trigonal bipyramidal structure was found. Likewise a total energy of -517.7036 a. u. with a C - F -distance of 2.58 a. u. in the tetraedral CF 4 and a CF 4 - F − -distance of 5.80 a. u. for the CF 4 -F − -system was found. The best C-F-distance of the tetraedral CF 4 was given at a distance of 2.51 a. u. According to this investigation it can be concluded that the structure of the system CF 4 - F − is the most stable one. An interesting result through the analysis of the electrondistribution is that the carbon atom is almost in all structure systems nearly unchanged.