METAL-COMPLEXES OF PROPANE-1,2,3-TRIAMINE - POTENTIOMETRIC AND CALORIMETRIC INVESTIGATIONS

The complexes formed by the ions Co2+, Ni2+, CU2+, and Zn2+ with the l igand propane-1,2,3-triamine (L-1) in 0.15 mol dm(-3) aqueous NaCl at 25 degrees C have been studied by potentiometry and calorimetry (ligan d to metal molar ratio, 1-3; pH range from 3.0 to 6.5-11.0). Stability constants have been obtained from potentiometric data by means of the least-squares program HYPERQUAD for the following species: [M(L-1)](2 +) and [MH(L-1)](3+) for all four metal cations; [M(L-1)(2)](2+) and [ MH(L-1)(2)](3+) for Ni2+, CU2+, and Zn2+; [MH2(L-1)(2)](4+) for Ni2+ a nd Cu2+; and [M(OH)(L-1)](+) for Cu2+ only. Stabilities increase in th e order Zn < Co < Ni < Cu. The complexes of L-1 have approximately the same stability as the corresponding ones formed by the related triami nes 2-methyl (2-methylamine) propane-1,3-diamine (L-3) and 2-ethyl (2- methylamine)propane- 1,3-diamine (L-4). The stability constants have b een used to obtain, from the calorimetric titration experiments, the f ormation enthalpies of all the metal complexes except those for cobalt (II) as well as the corresponding entropy changes. The thermodynamic q uantities are compared with the corresponding values previously found for L-3, in order to derive information about the coordination ability of the ligand. The analysis suggests that the unprotonated ligand for ms a fac-tridentate complex in most cases, but for some species, such as [Cu(L-1)(2)](2+), [CuH(L-1)(2)](3+), and [Zn(L-1)](2+), the thermod ynamic quantities indicate that at least one of the nitrogen atoms of the triamine is either loosely bonded or not coordinated to the metal ion. A gas-phase molecular mechanics analysis shows that the strain en ergies of the fac-[Co(L-1)(2)](2+) and -[Ni(L-1)(2)](2+) symmetrical ( s) and unsymmetrical (u) isomers are very similar. If the same solvati on energy is assumed for the two isomers, each isomer will be present in roughly equal amounts in solution.