CO capture at low concentration by catalysts is potentially useful for developing photocatalytic and electrocatalytic CO reduction systems. We investigated the CO-capturing abilities of two complexes, -Mn(Xbpy)(CO)(OCHCHNR) and -Re(Xbpy)(CO)(OCHCHNR) (Xbpy = 4,4'-X-2,2-bipyridine and R = -CHCHOH), which work as efficient catalysts for CO reduction. Both complexes could efficiently capture CO even from Ar gas containing only low concentration of CO such as 1% to be converted into -M(Xbpy)(CO)(OC(O)OCHCHNR) (M = Mn and Re). These CO-capturing reactions proceeded reversibly and their equilibrium constants were >1000. The substituents of Xbpy strongly affected the CO-capturing abilities of both Mn and Re complexes. The density functional theory (DFT) calculation could be used to estimate the CO-capturing abilities of the metal complexes in the presence of triethanolamine.