Abstract The structure of the metal chelate formed between Cu(II) and N-phosphonomethylglycine (PMG) has been determined by single crystal X-ray diffraction methods. The Cu(II) cation is five-coordinate with donor atoms in a distorted square pyramidal geometry for which the basal plane involves a tridentate combination of the carboxylate oxygen, one of the phosphonate oxygen atoms and the monoprotonated secondary amine nitrogen atom to form two adjacent chelate rings, together with a second phosphonate oxygen atom from an adjacent ligand. The complex is capped and further extended by coordination to one of the basal phosphonate oxygen atoms of yet another adjacent ligand. The phosphonate group of the ligand bridges between three Cu(II) centers through a monodentate O2 and a bidentate O1. Each Cu(II) cation is coordinated by all ligand donor atoms, carboxylate oxygen, nitrogen and phosphonate oxygen, in addition to the two bridging phosphonate oxygens of ligands which are primarily associated with adjacent chelate molecules. The Cu(II) complex is monoclinic, space group C2, with cell parameters a = 21.980(2), b = 4.779(2), c = 10.017(2) Å, β = 93.86(1)°, V= 1049.9(4) Å 3 and Z = 4. The structure was solved by direct methods and refined by a modification of direct difference and least-squares techniques to R = 0.072 with 715 independent reflections, I>3σ( I), and 62 variables.