Interplay between point symmetry, oxidation state, and the Kondo effect in 3 d transition metal acetylacetonate molecules on Cu(111)

We report that the occurrence of a Kondo effect in magnetic molecules crucially depends on the point symmetry and oxidation state of the adsorbed species. Two different transition metal acetylacetonate (acac) compounds [$M{(\mathrm{acac})}_{3}$, with $M=\mathrm{Cr}(\mathrm{III})$ or Co(III)] adsorbed on a Cu(111) single crystal were investigated to demonstrate the interplay. After deposition, $\mathrm{Cr}{(\mathrm{acac})}_{3}$ molecules formed threefold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{3}$ and twofold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{2}$ by releasing a ligand, while $\mathrm{Co}{(\mathrm{acac})}_{3}$ molecules only formed twofold symmetric $\mathrm{Co}{(\mathrm{acac})}_{2}$. Threefold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{3}$ molecules with a total electron spin $S=\frac{3}{2}$ exhibited no Kondo effect, while a clear Kondo resonance was observed in twofold $\mathrm{Cr}{(\mathrm{acac})}_{2}$ molecules. $\mathrm{Co}{(\mathrm{acac})}_{2}$ molecules surprisingly showed no Kondo resonance, even in the case of twofold symmetry, which is explained by a low-spin state of $S$ = 0. To analyze the results, a simple model is proposed based on the total electron spin and the symmetry of magnetic molecule. The present approach provides a feasible design strategy for single molecule magnets on metallic surfaces.