Size-Dependent Ligand Quenching of Ferromagnetism in Co 3 (benzene) n + Clusters Studied with X-ray Magnetic Circular Dichroism Spectroscopy.

Cobalt-benzene cluster ions of the form Co(bz) (n = 0-3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3-4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co has a spin magnetic moment of μ = 6 μ and an orbital magnetic moment of μ = 3 μ. Co(bz) and Co(bz) complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co. However, coordination of the third benzene to form Co(bz) completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co(bz) species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.