IR and 1 H-NMR studies on nitrilotriacetic acid (H 3 NTA) suggest that the acid exists in the zwitterion form, which allows the existence of intermolecular hydrogen bonding. A tetrahedral structure is established for eleven (1:1) anhydrous acid-metal (II) nitrilotriacetates complexes. The ten Dq values for the colored complexes were determined spectrophotometrically. The pK a values for the eleven acid metal complexes [M(HNTA)].(OH 2 ) 3 ] were determined and compared with the corresponding pK

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... values of the [M(OH 2 ) n ] +2 ions and also with the log β 1 values of the corresponding [M(NTA)]complexes. X-ray diffraction studies on the ligand and on eight of these complexes are described.