T. Matsubara et al., Ab initio MO study of the structures of N-2, NO, and CO molecules coordinated to the Pd-n (n=1, 2, 5, 25) clusters as a model of Pd(110) surface, J PHYS CH B, 105(16), 2001, pp. 3235-3244
The structures of N-2, NO, and CO molecules coordinated to the Pd-n (n = 1,
2, 5, 25) clusters as a model of Pd(110) surface were investigated by mean
s of ab initio MO methods. All three molecules, N-2, NO, and CO, coordinate
to the one Pd atom by the end-on mode, where the tilted end-on structure i
s more favorable for NO due to the additional electron in the pi* orbital.
On the contrary, in the Pd-2 system, N-2 coordinates to the bridge site of
Pd-2 cluster preferably by the side-on mode as reported previously (Blomber
g et al. Chem. Phys. Lett. 1991, 179, 524-530), although NO and CO prefer t
he end-on structure. On the other hand, on the hollow site of the Pd-5 clus
ter which is the (1 x 1) surface unit of Pd(110) with one Pd atom in the se
cond layer, the tilted structures with an unconventional coordination mode
with the interaction of the admolecule with the Pd atoms in both first and
second layers were found for all three molecules. With the further extensio
n of the cluster model from the Pd-5 cluster to the Pd-25 of the (3 x 3) su
rface unit of Pd(110) including Pd atoms up to the third layer, it was sugg
ested that the nonpolar N-2 and polar CO molecules favorably coordinate to
the top and the short-bridge sites, respectively, by the end-on mode in agr
eement with the experimental finding. Those coordination structures and sit
e preferences are discussed in terms of the electron donation and back-dona
tion between the Pd atoms and N-2, NO, and CO molecules.