Divalent platinum complexes of the carbanion 2-C₆F ₄AsPh₂: Monodentate or bidentate coordination?

The reaction of [PtI₂(1,5-COD)] with 2-LiC₆F ₄AsPh₂ affords the planar platinum(II) fluoroaryl complex [PtI(κC-2-C₆F₄AsPh₂)(1,5-COD)] (7), in which the arsenic atom is not coordinated to the metal atom. Attempts to prepare the bis(chelate) complex [Pt(κ²As,C-2-C₆F ₄AsPh₂)₂] analogous to the known phosphorus compound failed. Removal of iodide ion from 7 with TlPF₆ gave [Pt(κ²As,C-2-C₆F₄AsPh₂)(1,5- COD)]PF₆ (8), in which 2-C₆F₄AsPh₂ is now coordinated as a bidentate ligand, giving a four-membered chelate ring. Alternatively, protonolysis (with triflic acid) of the methyl complex [PtMe(κC-2-C₆F₄AsPh₂)(1,5-COD)] (15), which is obtained from 7 and dimethylzinc, gives the triflate salt 16 of the same chelate cation. The chelate As,C ring in 8 is readily reopened when the complex is treated with pyridine or with halide ions, forming respectively [Pt(py)(κC-2-C₆F₄AsPh₂)(1,5-COD)]PF ₆ (9) and [PtX(κC-2-C₆F₄AsPh ₂](1,5-COD)] (X = Cl (10), Br (11), I (7)). The appearance of two pairs of olefinic COD resonances in the proton NMR spectra of 7, 10, 11, and 15 may indicate that there is restricted rotation about the Pt-κC-2-C ₆F₄AsPh₂ bond; the additional signals are not evident in the spectra of 8 or 16. Complexes 7 and 10 form the 1:1 adducts [PtX(μ-2-C₆F₄AsPh₂)(1,5-COD)AuY] (X = Cl, Y = I (12); X = Y = I (13); X = Y = Cl (14)) by attachment of gold(I) halides to their dangling arsenic atoms. In 12 the halides are scrambled between platinum and gold. The X-ray structures of 7, 8, 9, 12, 15, and 16 are reported, and possible reasons for the poorer chelating ability of 2-C₆F ₄AsPh₂ in comparison with that of 2-C₆H ₄PPh₂ are discussed.