  We show that the magnetic state of individual manganese phthalocyanine (MnPc)
molecules on a Bi(110) surface is modified when the Mn2+ center coordinates to
CO molecules adsorbed on top. Using scanning tunneling spectroscopy we
identified this change in magnetic properties from the broadening of a
Kondo-related zero-bias anomaly when the CO-MnPc complex is formed. The
original magnetic state can be recovered by selective desorption of individual
CO molecules. First principle calculations show that the CO molecule reduces
the spin of the adsorbed MnPc from S = 1 to S = 1=2 and strongly modifies the
respective screening channels, driving a transition from an underscreened Kondo
state to a state of mix-valence.
