  The molecular self-assembly and the magnetic properties of two
cyclooctatetraenide (COT) - based single-ion magnets (SIM) adsorbed on Ag(100)
in the sub-monolayer range are reported. Our study combines scanning-tunneling
microscopy, X-ray photoemission spectroscopy and polarized X-ray absorption
spectroscopy to show that Cp*ErCOT (Cp* =
1,2,3,4,5-pentamethylcyclopentadienide anion) SIMs self-assemble as alternating
compact parallel rows including standing-up and lying-down conformations,
following the main crystallographic directions of the substrate. Conversely,
K[Er(COT)$_2$], obtained from subliming the [K(18-c-6)][Er(COT)$_2$]$\cdot$
2THF salt, forms uniaxially ordered domains with the (COT)$^{2-}$ rings
perpendicular to the substrate plane. The polarization-dependent X-ray
absorption spectra reproduced by the multiX simulations suggest that the strong
in-plane magnetic anisotropy of K[Er(COT)$_2$]/Ag(100) and the weak
out-of-plane anisotropy of Cp*ErCOT/Ag(100) can be attributed to the strikingly
different surface ordering of these two complexes. Compared to the bulk phase,
surface-supported K[Er(COT)$_2$] exhibits a similarly large hysteresis opening,
while the Cp*ErCOT shows a rather small opening. This result reveals that
despite structural similarities, the two organometallic SMMs have strongly
different magnetic properties when adsorbed on the metal substrate, attributed
to the different orientations and the resulting interactions of the ligand
rings with the surface.
