A Density Functional Theoretical Study Of M₂₀ (PMe₃)₄ (M=Cu, Ag, Au)

Study of metal clusters with organic ligands is always of great interest inbond-activation processes and catalysis. Transition metal clusters play acentral role in both homogeneously and heterogeneously catalysis.Photoelectrons spectroscopy revealed that Au₂₀ cluster has an extremely largefrontier orbital energy gap which was even greater than that of C60. Thatobservation suggested that Au₂₀ cluster should be highly stable and maypossess novel chemical and physical properties. It was found that the moststable geometries of Ag₂₀ and Au₂₀ are tetrahedral Td structures, while Cu₂₀preferred a compact structure with Cs symmetry. In this paper, we will keepthe Td symmetry for all M₂₀ clusters (M = Cu, Ag, Au).Ligands play a crucial role to make metal cluster stable. Phosphane PR3 isone of the most ubiquitous ligands in transition-metal chemistry. Zhang andco-workers synthesized the tetrahedral Au₂₀ cluster coordinated with eight PPh3 (Ph=phenyl) ligands in solution. They found that there were four PPh3ligands which could be easily removed from Au₂₀(PPh3)82+, resulting in ahighly stable Au₂₀(PPh3)42+ ion. The observation was consistent withtetrahedral Au₂₀ in which four apex sites are expected to bond to the PPh3ligands strongly, and it is confirmed by theoretical calculations which predicta highly stable Au₂₀(PH3)4 complex with Au-PH3 bond energies of ¹eV.Metal complexes with phosphane ligands have been widely used as powerfulcatalysts in homogeneous catalysis which are important for industrialpurposes.Theoretical research about clusters of M₂₀(PMe₃)4 (M = Ag, Cu, Au) havenever been reported. To understand the properties and nature of the complexeswe made a further investigation on the interactions between PMe₃ ligated andM₂₀ (M= Cu, Ag, Au) metal clusters. In this work, complexes M₂₀(PMe₃)4(M=Cu, Ag, Au) with Td structure have been studied by means of ab-initio(HF, MP2) and DFT methods. M₂₀(PMe₃)4 with PMe₃ in two kinds ofcoordination situation was compared. Some conclusions may be summarizedas follows.1) The coinage M₂₀ clusters in Td symmetry all have great HOMO-LUMOgap while Au₂₀ has the biggest one. The gap of Au₂₀ is even larger than thatof C60 compound. The great gap to some extent reflects the stability of this system.2) M₂₀ (M = Cu, Ag) clusters protected by PMe₃ ligands become more stable,while Au₂₀ becomes less. Anyhow Au₂₀(PMe₃)₄ still has a largeHOMO-LUMO gap more than 1.5 eV which still comparable to C60.3) The ligand adsorbed on top position is more stable than on face-centerposition in all M₂₀ systems. The sequence of adsorption energy betweenM₂₀ and 4(PMe₃) is Au>Cu>Ag.