Square-planar copper(II) complexes with a novel tetradentate amido-carboxylate ligand.: Crystal structure of [Co(H2O)6][Cu(mda)] • 2H2O

A novel O-N-N-O-type tetradentate ligand H(4)mda (H(4)mda = malamido-N,N'-diacetic acid) and the corresponding square-planar copper(II) complexes have been prepared and characterized. The mda(4-) ligand coordinates to the copper(II) ion via two pairs of deprotonated ligating atoms (two carboxylate oxygens and two deprotonated amide nitrogens) with in-plane square chelation. A four-coordinate, square-planar geometry has been established crystallographically for the [Co(H2O)(6)][Cu(mda)] . 2H(2)O complex. Structural data correlating the square-planar geometry of the [Cu(mda)](2-) unit are discussed in relation to information obtained for similar complexes. The i.r., electronic, absorption and reflectance spectra of the complexes are analysed in comparison with related complexes of known geometries.