DFT Study of Small Palladium Clusters Pd-n and Their Interaction with a CO Ligand (n=1-9)

This paper presents a theoretical study of palladium clusters Pd-n of low nuclearity, ranging from 1 to 9, completed by their interactions with a carbonyl ligand (PdnCO). The simulations were performed at the DFT (B3LYP-Lanl2DZ) level. Small palladium clusters are deltahedral structures characterized by a triplet ground state (Pd-2 to Pd-7) or a quintet ground state for larger nuclearities (Pd-8 and Pd-9). Various electronic states relatively close in energy are present. These states have close geometries but differ by their electron density distribution. A significant electronic population of the 5s orbital is needed to explain the existence and cohesion of palladium-palladium bonds. These latter are formed by the combination of sd hybrid orbitals and get stronger as the 5s character increases. The relative stability of palladium clusters increases with their nuclearity and the study of fragmentation reactions shows a particular stability of Pd-4, Pd-6 and Pd-9 clusters. The adsorption of the CO ligand goes through a back-bonding interaction favoured by a high 4d and a low 5s atomic population, This interaction therefore affects the metal-metal bond and the ground state of the cluster becomes a singlet for the first members of the series. The favoured coordination mode is one that maximizes the back-bonding interaction, i.e. the capped position (mu(3)) on a cluster's face. However, stabilization provided by the presence of one ligand at the expense of a large number of metal-metal bonds is not sufficient and, in the case of larger clusters, a triplet ground state remains. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)