The formation of ion-pair adducts between the cationic complex La(THP)(3+) (THP = 1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane) and the anionic complexes Tm(DOTA)(-) (DOTA = 1,4,7,10-tetraazacyclododecaneNN',N",N"'-tetraacatate), Tm(DTPA)2(-) (DTPA = diethylenetriamine-N,N,N',N",N"-pentaacetate), Tm(TTHA)(3-) (TTHA = triethylenetetraamine-N,N,N',N",N"',N"'-hexaacetate), and Tm(DOTP)(5-) (DOTP = 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetrakis(methylenephosphonate)) is examined by C-13 NMR spectroscopy. The induced 13C shifts Of the La(THP)3+ complex are followed by titration of the Tm(III) complexes of DOTA, DTPA, and TTHA at pH 7. From these data, the stability constants are calculated to be beta (1) = 64 M-1 (1:1), beta (1) = 296 M-1 (1:1), and beta (2) = 26 000 M-2 (2:1) for the ion pairs of La(THP)(3+), with Tm(DOTA)(-), Tm(DTPA)(2-), and Tm(TTHA)(3-), respectively. The La(THP)(3+),Tm(DOTP)(5-) system elicits chiral resolution of the rapidly interconverting Tm(DOTP)(5-) isomers.