Page 225 - Adsorbents fundamentals and applications

210 π-COMPLEXATION SORBENTS AND APPLICATIONS

Table 8.5. Changes in electron occupancies upon adsorption ( Oc) in the outer-shell
orbitals of Ag
Orbital 5s 4d xy d xz 4d yz 4d x2−y2 4d z2 4d (5s + 4d)

C 2 H 4 -AgF 0.1269 −0.0001 −0.0003 −0.0291 −0.0004 −0.0421 0.0450 0.0819
C 2 H 4 -AgCl 0.1204 −0.0001 −0.0004 −0.0205 −0.0003 −0.0258 −0.0470 0.0734
C 2 H 4 -AgI 0.1091 −0.0001 −0.0004 −0.0183 −0.0003 −0.0219 −0.0410 0.0681
C 2 H 4 -AgZ 0.0596 −0.0016 −0.0089 −0.0032 −0.0011 −0.0029 −0.0177 0.0419
Chen and Yang, 1996.

molecule and for Ag in the adsorbents. Occupancies before and after adsorption
are given. The changes in occupancy upon adsorption ( Oc) are also given in
Table 8.5. It is obvious from Tables 8.4 and 8.5 that, upon adsorption, the electron
occupancy of the 5s orbital of Ag always increases, whereas the total occu-
pancy of its 4d orbitals (4d xy ,4d xz ,4d yz ,4d x2−y2 ,and 4d z2 ) always decreases.
This result is caused by the σ-donation from the π-bond (i.e., mainly 2P y and
2P z orbitals) of the adsorbate molecule to the 5s orbital of Ag, and the d-
∗
∗
∗
π backdonation from the 4d orbitals of Ag to the π -bond (2P ,2P ,and
∗
x y
2P ∗ orbitals, also shown in the 2P orbitals) of the adsorbate. This is in line
z
with the classic picture of Dewar (1951) and Chatt and Duncanson (1953) for
π-complexation.
The net increase in occupancy indicates a net electron transfer from the adsor-
bate to the Ag-containing adsorbent. The net increase in the occupancy of the
5s orbital of Ag indicates the strength of the forward σ-donation bond. The net
decrease in the occupancies of the 4d orbitals of Ag indicates the strength of
∗
the d-π backdonation bond. The ratios of Oc of 5s over the Oc of 4d are
approximately 2 : 1 to 3 : 1, indicating the relative contributions of the σ-donation
over the d-π backdonation to the overall bond.
∗
It is interesting to compare the electron occupancies of the Ag atom in zeo-
lite and different halides. The occupancy in the 5s orbital follows the order of
−
electronegativities of the halides, that is, being the lowest with F , which has
the highest electronegativity. The occupancy in the Ag-zeolite model is clearly
lower than that in AgF, indicating that the zeolite framework anion is highly
electronegative.
A careful examination of the occupancy changes in the 4d orbitals of Ag
reveals an interesting pattern. The occupancy changes for 4d xy ,4d xz ,and 4d x2−y2
are small, while the major changes occur in the 4d yz and 4d y2 orbitals. The
∗
overlap between the 4d yz orbital of Ag with the 2p orbitals of the adsorbate
molecule follows the classic picture of π-complexation, illustrated in Figure 8.4.
However, the large occupancy decreases of the 4d yz orbitals are unexpected. The
three other 4d orbitals (4d xy ,4d xz ,and 4d x2−y2 ) are positioned perpendicular
in relation to 4d yz . Thus, there is no possibility for them to overlap with 4d yz .
The 4d z2 orbitals, or the dumbbell-and-doughnut shaped orbitals, also shown
in Figure 8.4, are in the vicinity of the spatial direction of the 4d yz orbitals.
As a result, the 4d z2 orbitals are the only ones that can overlap with the 4d yz