Arsenic tetrahedral geometry

Complexes with phosphorus or arsenic as the central atom have been synthesized from silylpricogenides and [Au3(p,-0)(PR3)3]+ or [Au(PR3)]BF4 [95, 105-108]. As commented above, the structure of [Au4( j,-As)(PPh3)4]+ does not display the classical tetrahedral geometry towards the As center [95]. The arsenic atom is situated in the apex of a square pyramid, with the lone electron pair allocated to the As apex. This fact would be in agreement with other situations in which the final geometry of the molecules is influenced by the aurophilic attraction, and supports the idea that the stabilization of the final compound is based on the presence of aurophilic interactions which probably are less favorable in a tetrahedral geometry for arsenic, in comparison with nitrogen, due to the different size of these atoms. 

An arsenic-arsenic bond (2.427 A) is generated when dimethylarsenic iodide reacts with Ga2l4 to give an ionic product, formulated as [Me2As—AsMejIlCGalJ . The cation contains both As(III) and As(V) the former is pyramidal (C—As—C 99° with normal bond distances (1.982,2.028 A), while at arsenic(V), the geometry is distorted tetrahedral (C—As—C 105-114°) and the As—C distances are substantially shorter (1.916,1.833 A). 

The aluminium compound, (Et2AlAsBu2)2, obtained from triethylaluminium and dibutylarsine by loss of ethane contains a four-membered ring (As—A12.57 A) and, as shown in Table 3, four-membered, often planar and centrosymmetric rings dominate the related arsenic-gallium species 62. The geometry at each of the ring atoms is distorted tetrahedral. 

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