We present a theoretical investigation of cold reactive and non-reactive collisions of Li and Rb atoms with C − 2 . The potential energy surfaces for the singlet and triplet states of the Li-C − 2 and Rb-C − 2 systems have been obtained using the CASSCF/ic-MRCI+Q approach with extended basis sets. The potential energy surfaces are then used to investigate the associative detachment reaction and to calculate rotationally inelastic cross sections at low collision energies by means of the close-coupling method. The results are compared to those obtained for other anionic systems such as Rb-OH − , and the implications for hybrid trap experiments and sympathetic cooling experiments are explored. Furthermore, we discuss the possibility to perform Doppler thermometry on the C − 2 anion and investigate the collision process involving excited electronic states.