WEAK CURIE-TYPE PARAMAGNETISM OF HIGH-NUCLEARITY PLATINUM CARBONYL CLUSTERS - ANOMALOUS MAGNETIC-BEHAVIOR

This paper reports the magnetic behavior of large platinum carbonyl clusters: (Bu4N)2[Pt6(CO)12] (1), (Bu4N)2[Pt9(CO)18] (2), (Bu4N)4[Pt19(CO)22] (3), and (PPN)2[Pt38(CO)44Hx] (4). We found these platinum clusters to be "diamagnetic" at room temperature but "weakly paramagnetic" at low temperature. The observed paramagnetism (which is an anomaly considering that these are all even-electron clusters) can be described by the simple Curie law. The Van Vleck temperature-independent paramagnetic susceptibility χp increases with cluster size. Such marked increase in χp with increasing cluster size can be attributed to the decrease in the energy gap between the occupied states and the unoccupied states as the cluster size increases. Another important result is that the effective magnetic moment, μeff, falls in the narrow range of 0.35-0.55 μB per cluster. From the observed magnetic moments, we calculated the μeff value per atom to be 0.1 ± 0.02 μB, a value very close to the corresponding value of 0.09 μB observed by Marzke et al. for platinum crystallites containing approximately 400 atoms. The fact that these molecular platinum clusters have magnetic properties very similar to (indeed approaching) those of the small crystallites is a strong indication that these clusters begin to enter the quantum-size regime where the magnetic properties are distinctly different from those of the bulk as well as those of the molecules.