C-13 NMR structure of the palladium complexonates in aqueous solutions

Aqueous solutions of palladium complexonates with iminodiacetic acid H(2)Id a (IDA), nitrilotriacetic acid H(3)Nta (NTA), ethylenediamine-N,N'-diacetic acid H(2)Edda (EDDA), and ethylenediamine-N,N,N',N'-tetraacetic acid H(4)E dta (EDTA) {[Pd(Ida)(2)](2-), [Pd(Nta)(2)](4-), [Pd(Edda)(2)](2-), and [Pd( Edta)](2-), respectively} were studied by the C-13 NMR and IR spectroscopy methods. It was established with the use of a paramagnetic gadolinium compl ex that palladium in the bis(IDA) and bis(NTA) complexes is symmetrically c oordinated by two glycine fragments, each from one of the two complexone mo lecules, and its coordination number (C.N.) is 4. The palladium coordinatio n sphere in the bis(EDDA) complex incorporates four nitrogen atoms and two extra coordinated oxygen atoms (of the two different ligands), resulting in C.N. 4 + 2. In the [Pd(Edta)](2-) complexonate, palladium is coordinated b y two glycine fragments, each from one of the ligand's halves, thus retaini ng symmetry of the EDTA ligand. The remaining two acetate branches of the i minodiacetate fragments are not involved in the coordination. At reduced pH , the protonation was found to occur precisely at the "free" carboxylate gr oups of EDTA. A comparison of the IR and NMR data indicates that the resolv ing ability of vibrational spectroscopy in the 1600 cm(-1) region is insuff icient to discriminate between the coordinated and free COO groups of the [ Pd(Ida)(2)](2-), [Pd(Nta)(2)](4-), and [PdEdta](2-) complexes, whereas thes e fragments are distinguishable in the C-13 NMR spectra and can be identifi ed by applying the paramagnetic contrasting technique.