Importance of the Apical Site of the (H2PC2H4PH2)Pd Complex on the Elementary Reactions. A Density Functional Study

Abstract

The importance of the apical site of the palladium complex on the elementary reactions is theoretically examined with a density functional method (B3LYP) using the model complex (H 2 PC 2 H 4 PH 2 )Pd. It is well-known by many theoretical analyses that the oxidative addition and reductive elimination of the chemical bonds, such as H-H and C-H, on the phosphine-coordinated palladium complex, (PH 3 ) 2 Pd, take place in the equatorial plane by the charge transfers between the d and sp orbitals of the Pd enhanced by the equatorial phosphine ligands and the bonding and antibonding orbitals of the chemical bonds. Therefore, the role of the fifth site at the apical position has not been known so far. In the present study, we successfully suggested the important role of the apical site to significantly stabilize the potential surface of the reaction and lower the energy barrier, on the oxidative addition of the C-X (X = Sn, Ge, Si, C) bonds of heteroles to the Pd of (H 2 PC 2 H 4 PH 2 )Pd and the insertion of XH 2 into the Pd-C bond of (H 2 PC 2 H 4 PH 2 )Pd(η2-HC≡CH). In the insertion reaction, the effect of the apical site increases in the order Sn Ge > Si > C, which indicates that the importance of the apical site increases in the case for the strongly polarized C-X bond. However, the apical site was invalid for the oxidative addition of the C-X bond of CH 3 XH 3 , which has no π orbital on the C-X carbon promoting the charge transfer on the apical site. The possibility of the intramolecular rearrangement of the (H 2 PC 2 H 4 PH 2 )Pd(H)(XH 3 ) complex which switches the position of the hydrido and XH 3 ligands utilizing the apical site was also suggested.