The invention relates to a chelate resulting from the complexing of a trans-di-N-picolinate tetraazacycloalkane ligand with a metal cation, said ligand corresponding to formula (A) wherein n is equal to 0 or to 1, R is H or a C1-C18 alkyl radical, and R' is H or a C1 -C18 alkyl radical, said metal cation being a cation of a metal selected from the group consisting of lead (II) and bismuth (III). The invention is applicable in the field of lead trapping and alpha-radioimmunotherapy.