  We report a combined experimental and theoretical study of the Kondo effect
in a series of binuclear metal-organic complexes of the form
[(Me(hfacac)_2)_2(bpym)]^0, with Me = Nickel (II), Manganese(II), Zinc (II);
hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on
Cu(100) surface. While Kondo-features did not appear in the scanning tunneling
spectroscopy spectra of non-magnetic Zn_2, a zero bias resonance was resolved
in magnetic Mn_2 and Ni_2 complexes. The case of Ni_2 is particularly
interesting as the experiments indicate two adsorption geometries with very
different properties. For Ni_2-complexes we have employed density functional
theory to further elucidate the situation. Our simulations show that one
geometry with relatively large Kondo temperatures T_K ~ 10K can be attributed
to distorted Ni_2 complexes, which are chemically bound to the surface via the
bipyrimidine unit. The second geometry, we assign to molecular fragmentation:
we suggest that the original binuclear molecule decomposes into two pieces,
including Ni(hexafluoroacetylacetonate)_2, when brought into contact with the
Cu-substrate. For both geometries our calculations support a picture of the
(S=1)-type Kondo effect emerging due to open 3d shells of the individual
Ni^{2+} ions.
