H5XT (bis{[bis(carboxymethyl)amino]methyl}phosphinic acid) is an EDTA(4-)-like ligand containing an extra phosphinate group. [Co-II(XT)](3-), [Co-III(XT)](2-), and a series of [Ln(XT)](2-) complexes have been prepared. The phosphinate group is not coordinated in the Co complexes but is bound in the lanthanide complexes. Solid state and solution behaviors of Ln-XT species are consistent: both monoprotonated and nonprotonated species have been found. Protonation of the metal complex does not lead to dissociation of a carboxylate; rather, the proton distributes around the molecular ion. The pM values of Ln-XT are comparable to those of Ln-EDTA but are higher than those of Ln-TMDTA. The inclusion of a phosphinate eases the selectivity of an EDTA-type ligand for late lanthanides.