Change of coordination from tetrahedral gold-ammonium to square-pyramidal gold-arsonium cations

RECENT work^1-5 has shown that gold-based ligands in compounds of carbon and nitrogen can induce novel molecular structures with coordination numbers at C and N as high as 5 and 6. These phenomena must be ascribed to metal-metal interactions (Au...Au), which can overrule bonding in classical configurations. Here we describe a study of the molecular structures of tetra(auro)ammonium ((LAu)₄N⁺) and tetra(auro)arsonium ((LAu)₄As⁺) cations (where L is a ligand). We find that the classical tetrahedral structure in these four-coordinate compounds is abandoned in favour of a square-pyramidal geometry once the radius of the central element is too large to allow for metal-metal bonding in a tetrahedral geometry. Thus, whereas the nitrogen compounds adopt a tetrahedral structure, for the larger arsenic atom an arsenic-capped square of gold atoms represents a more favourable core geometry. We have not yet been able to prepare the intermediate phosphorus compound, but we expect it also to have the square-pyramidal structure.