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