$symmetric orientation of the biden-
tatepair to the metal ions. In many multidentate ligands such binding presents steric
requirements that are frequentlydifficult to meet, as discussed below.
In addition to well known combinations of amino and carboxylate groups, special attention
will be given to phosphonate, phenolate, hydroxamate, catecholate and peptide donors.
Table 3 presents representative data that are currently available (Ref. 4) on the stabilities
of metal chelates of a series of ligands that constitute a linear extension of the NTA and