Size-Dependent Ligand Quenching of Ferromagnetism in Co₃(benzene)_n⁺ Clusters Studied with X-ray Magnetic Circular Dichroism Spectroscopy

Cobalt-benzene cluster ions of the form Co₃(bz)_n⁺ (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. Co₃⁺ has a spin magnetic moment of μ_S = 6 μ_B and an orbital magnetic moment of μ_L = 3 μ_B. Co₃(bz)⁺ and Co₃(bz)₂⁺ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co₃⁺. However, coordination of the third benzene to form Co₃(bz)₃⁺ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co₃(bz)_n⁺ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.