Stability constants of the mixed-ligand copper(II)-1, 10'-phenanthroline-amino acid complexes studied by potentiometric measurements

We studied the stability constants of the mixed-ligand copper(II) complexes formed with 1,10'-phenanthroline (Phen) and the amino acids valine (HVal, HL), leucine (HLeu, HL), isoleucine (HIle, HL), tripthophan (HTrp, HL), asparagine (HAsn, HL) and glutamine (HGln, HL) in 1.0 mol L-1 KNO3 aqueous solution at 25 degrees C. Potentiometric data were analyzed with the Letagrop program. The complexes [Cu(Phen)(H2L)](3+), [Cu(Phen)(HL)](2+), [Cu(Phen)(L)](+), Cu(OH)(Phen)(L) and Cu(Phen)(L)(2) were observed in the different systems studied. The species distribution diagrams as a function of pH are briefly discussed. The relative stability parameters D log K and the relative stability % R.S. values were determined for the ternary complexes and were compared to the stability constants of the binary ones. In an effort to understand the possible structures formed in solution, molecular absorption spectra were analyzed around the 600 nm region, which indicated the formation of five-coordinate complexes likely with square pyramidal structures.