Predictions of two- and three-body negative ion states

A peculiar class of weakly bound states is discussed. These states are made up of three entities which have the property that: A does not form bound states with B or C seperately but there is a BC bound state and an ABC bound state. The term Tango state is suggested to distinguish this class from other weakly bound systems. Two mechanisms for forming the ABC Tango state are discussed. The following systems are predicted to form bound states: H− 3 , HeH, NeH, ArH, ... and Mg− 2 ; quantum chemistry calculations very strongly indicate H− 3 and HeH form bound states. For the first four species any exchange of D for H will also be a Tango state. 36.90.+f, 31.20.Pv, 31.20.Tz Typeset using REVTEX