Density Functional Analysis of Geometries and Electronic Structures of Gold-Phosphine Clusters. The Case of Au4(PR3)42+ and Au4(μ2-I)2(PR3)4

Geometries, ligand binding energies, electronic structure, and excitation spectra are determined for Au-4(PR3)(4)(2+) and Au-4(mu(2)-I)(2)(PR3)(4) clusters (R = PH3, PMe3, and PPh3). Density functionals including SVWNS, X alpha, OPBE, LC-omega PBE, TPSS, PBE0, CAM-B3LYP, and SAOP are employed with basis sets ranging from LANL2DZ to SDD to TZVP. Metal-metal and metal-ligand bond distances are calculated and compared with experiment. The effect of changing the phosphine ligands is assessed for geometries and excitation spectra. Standard DFT and hybrid ONIOM calculations are employed for geometry optimizations with PPh3 groups. The electronic structure of the gold-phosphine clusters examined in this work is analyzed in terms of cluster ("superatom") orbitals and d-band orbitals. Transitions out of the d band are significant in the excitation spectra. The use of different basis sets. and DFT functionals leads to noticeable variations in the relative intensities of strong transitions, although the overall spectral profile remains qualitatively unchanged. The replacement of PMe3 with PPh3 changes the nature of the electronic transitions in the cluster due to low-lying pi*-orbitals. To reproduce the experimental geometries of clusters with PPh3 ligands, computationally less expensive PH3 or PMe3 ligands are sufficient for geometry optimizations. However, to predict cluster excitation spectra, the full PPh3 ligand must be considered.