A Novel Anionic Gold-Indium Cluster Compound: Synthesis and Molecular and Electronic Structure

The insertion of InBr into the Au - Br bond of [(Ph₃P)AuBr] in tetrahydrofuran (thf) in the presence of [(CH₂-PPh₂)₂] (dppe) leads to the formation of an orange complex [(dppe)₂Au]⁺[(dppe)₂Au₃In ₃Br₇(thf)]^-, 2. Analytical, spectroscopic, and X-ray structural investigations showed that this product is an anionic analogue of a neutral chloride complex [(dppe)₂Au₃In₃Cl₆(thf)₃], 1, prepared recently. Both complexes have an Au₃In₃ cluster core of approximate C_2v symmetry with one extremely short Au-Au bond [Au1-Au3 2.575(1) Å] as part of a quasilinear array P1- Au1- Au3-P4, suggesting the presence of a bis(phosphine) complex of the neutral Au₂ molecule as part of the cluster. The third gold atom (Au2) is then assigned oxidation state +1. To gain deeper insight into the structure and bonding of this novel class of gold cluster compounds, regarding mainly the peculiar cluster geometry, the charge distribution, and the oxidation states, a series of scalar relativistic all-electron density functional (DF) calculations on model systems has been performed. As a model for 1, the neutral cluster {Au₃(PH₃)₄[InCl₂(H ₂O)]₃} was studied. For the examination of the geometry of complexes 1 and 2, the cluster Au₃(PH₃),₄I₃ has been considered as a further simplified model, where iodine replaces the InX₂(thf) units. Experimental and calculated cluster geometries agree satisfactorily, and the formal oxidation states of the gold atoms (0 for Au1 and Au3, +1 for Au2) could be confirmed, but for the In centers no interpretable differences of the Mulliken charges were found.