ABSTRACT
An entirely different situation exists on the (010) crystal plane. As mentioned before, in the strong OTI bond of terminal Mo=Ot on the (010) plane there is sufficient transfer of electron density onto the metal orbitals to render the terminal oxygen 0t only weakly basic. Conversely, the exposed bridging oxygens are more basic and their lone pairs are capable of performing a nucleophilic attack on an organic molecule. These electron pairs form the HOMO at the surface of Mo03. Its LUMO is the Mo 4d [71], but this orbital is relatively inaccessible to the reactants of the catalytic reaction because it is screened by the close-packed surface oxygen atoms. The HOMO and LUMO of allyl species are illustrated in Fig. 7 .17 . The HOMO in the allyl cation is the bonding three-nuclear TI orbital, the LUMO consists of the nonbonding TI orbital. The reaction between allyl species and Mo03 to form a chemisorbed species is governed by HOMO - LUMO interactions, of which there are two possibilities: (1) an electrophilic attack by the catalyst Mo 4d empty orbital on the allyl radical and (2) a nucleophilic attack by the surface bridging oxygen lone pair orbitals on the empty TI orbital of allyl. In view of the inaccessibility of the Mo 4d orbitals and the more pronounced electron acceptor properties of the allyl cation or allyl radical, pathway 2 seems to be more probable, all conditions of the swift reaction being fulfilled.
