The magnetic properties of metal clusters are strongly influenced by their size and surface ligands. In general, the magnetic moments in small metal clusters are larger than in the case of a macroscopic bulk metal structure. For example, the average magnetic moment per atom in Ni clusters was found to be 0.7-0.8 μB, as compared with 0.6 μB for bulk Ni. This is explained by longer metal–metal bonds in cluster structures than in bulk structures, a consequence of a larger s character of metal–metal bonds in clusters. Magnetic moments approach bulk values as cluster size increases, though this is often difficult to predict computationally.
Magnetic quenching is an important phenomenon that is well documented for Ni clusters, and represents a significant effect of ligands on metal cluster magnetism. It has been shown that CO ligands cause the magnetic moments of surface Ni atoms to go to zero and the magnetic moment of inner Ni atoms to decrease to 0.5 μB. In this case, the 4s-derived Ni–Ni bonding molecular orbitals experience repulsion with the Ni-CO σ orbital, which causes its energy level to increase so that 3d-derived molecular orbitals are filled instead. Furthermore, Ni-CO π backbonding leaves Ni slightly positive, causing more transfer of electrons to 3d-derived orbitals, which are less disperse than those of 4s. Together, these effects result in a 3d, diamagnetic character of the ligated Ni atoms, and their magnetic moment decreases to zero.
Density functional theory (DFT) calculations have shown that these ligand-induced electronic effects are limited to only surface Ni atoms, and inner cluster atoms are virtually unperturbed. Experimental findings have described two electronically distinct cluster atoms, inner atoms and surface atoms. These results indicate the significant effect that a cluster's size has on its properties, magnetic and other.