DIFFERENTIAL SCANNING CALORIMETRIC INVESTIGATIONS OF LARGE TRANSITION-METAL CLUSTERS

The thermal behavior of various ligand stabilized transition metal clusters is investigated by means of differential-scanning-calorimetry (DSC). In the case of the cluster Au-55(PPh(3))(12)Cl-6 the decomposition temperature of 131 degrees C, the decomposition enthalpy of 114 J/g, as well as the bond energy of 75.6 kJ/mole can be determined. Moreover, detailed informations about the mechanism of cluster degradation into Ph(3)PAuCl, (Ph(3)P)(2)AuCl, PPh(3), and metallic gold can be made in combination with impedance spectroscopic measurements. The derivatives Au-55[P(p-tolyl)(3)]Cl-12(6) and Au-55[P(p-anisyl)(3)]Cl-12(6) behave similar like the PPh(3) substituted clusters. The thermolysis of the water-soluble cluster Au-55(TPPMS-Na)(12)Cl-6 proceeds completely different, what can be attributed to the formation of (TPPMS-Na)Au(TPPMS). The DSC diagram of the platinum cluster Pt(309)phen*O-36(30) shows no decomposition signal between 25 and 400 degrees C although degradation occurs. This must be due to compensating processes of unknown nature. The thermal behavior of the larger 5-, 7-, and 8-shell palladium clusters documents the relation to the metallic state. The points of decomposition are found between 130 and 115 degrees C, the decomposition enthalphies between 150 and 60 J/g, and the bond energies between 58-56 kJ/mole what is slightly smaller than in the bulk metal with 62.8 kJ/mole.