Fig. illustrates the design principle. We first consider cations of third group elements (yttrium or scandium). These metals lose one of their three valence electrons by bonding to a 17 th group atom, which requires one electron to reach a closed-shell configuration. To make a cation, the second valence electron is removed leading to a molecule with a positive charge and the desired unpaired electron on the metal. We follow the same strategy to construct other pairs of a metal with n valence electrons and a ligand missing n -2 electrons to a closed-shell, as shown in Fig. .