“…[29] Cationic metal-CO 2 complexes M + (CO 2 ) n (M = Mg, Al, Si, V, Fe, Co, Ni, Rh, Ir) have been exten-sively investigatedi nt he past decades, [30][31][32][33][34][35][36][37][38][39][40] and also anionic speciesM À (CO 2 ) n (M = Ti,M n, Fe, Co, Ni, Cu, Ag, Au, Sn, Bi) have received considerable attention,f oremost by the group of Weber. [41][42][43][44][45][46][47][48][49][50][51] Generally,t he anionic CO 2 À stretching vibrations shift to the red compared to neutral CO 2 vibrations. [4,52] CO 2 as al igand was also investigated as metal oxides, NbO 2 + (CO 2 ) n and Ta O 2 + (CO 2 ) n by Mackenzie and co-workers.…”