The binding of propylene oxide (PO) to a series of metal cations LM(III)(+), where for L = tetraphenylporphyrin (TPP) M =Al, Ga, Cr, and Co, and for L = (R,R)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine (salen) M = Al and Cr, was studied in the gas phase by electrospray tandem mass spectroscopy, and the relative stabilities of LM(PO)(2)(+) and LM(PO)(+) cations were determined. The chromium(III) and aluminum(III) cations most tenaciously bind PO, and for M = Al, coordination to the TPP ligated metal center was favored relative to salen. For (TPP)M(PO)(2)(+), the dissociation of PO followed the order M = Al > Cr, but for (TPP)M(PO)(+) the dissociation was M = Cr > Al. The single-crystal structural determinations on (R,R-salen)AlOCHMe(S)CH(2)Cl(.)0.5PO and (R,R-salen)AlO(2)CMe(.)1.5py grown in neat PO and pyridine, respectively, reveal five-coordinate aluminum(III) centers with the alkoxide/acetate ligands in the axial position of a square-based pyramid. These results are discussed in terms of the reactivity of these metal complexes in ring-opening polymerizations and copolymerizations with PO and CO2, respectively.