The present invention relates to dimeric pentadentate chelators with exceptionally strong binding of metal ions, for detection, immobilization and purification of biomolecules. Dimeric chelators offer a cooperativity of binding of two adjacent immobilized metal ions simultaneously to a histidine-tagged biomolecule, which gives advantageous properties regarding strength of binding compared to a corresponding monomer chelator. In addition, a dimer increases the selectivity (ease of separation) against non-tagged biomolecules with low metal-ion affinity.