Superweak Complexes of Tetrahedral P4 Molecules with the Silver Cation of Weakly Coordinating Anions

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Abstract
The silver aluminates AgAl[OC(CF3)2(R)]4 (R=H, CH3, CF3) react with solutions of white phosphorus P4 to give complexes that bind one or two almost undistorted tetrahedral P4 molecules in an η2 fashion: [Ag(P4)2]+[Al(OC(CF3)3)4]− (1) containing the first homoleptic metal–phosphorus cation, the molecular species (P4)AgAl[OC(CH3)(CF3)2]4 (2), and the dimeric Ag(μ,η2-P4)Ag bridged {(P4)AgAl[OC(H)(CF3)2]4}2 (3). Compounds 1–3 were characterized by variable-temperature (VT) 31P NMR spectroscopy (1 also by VT 31P MAS-NMR spectroscopy), Raman spectroscopy, and single-crystal X-ray crystallography. Other Ag:P4 ratios did not lead to new species, and this observation was rationalized on thermodynamic grounds. The Ag(P4)2+ ion has an almost planar coordination environment around the Ag+ ion due to d(Ag)→σ*(P−P) backbonding. Calculations (HF-DFT) on six Ag(P4)2+ isomers 4 a–f showed that the planar η2 form 4 a is only slightly favored by 5.2 kJ mol−1 over the tetrahedral η2 species 4 b; η1-P4 and η3-P4 complexes are less favorable (27–76 kJ mol−1). The bonding of the P4 moiety in [RhCl(η2-P4)(PPh3)2], the only compound in which an η2 bonding mode of a tetrahedral P4 molecule has been claimed, must be regarded as a tetraphosphabicyclobutane, and not as a tetrahedro-P4 complex, on the basis of the published NMR and vibrational spectra, the calculated geometry of [RhCl(P4)(PH3)2] (10), the highly endothermic (385 kJ mol−1) calculated dissociation enthalpy of 10 into P4 and RhCl(PH3)2 (11), as well as atoms in molecules (AIM) and natural bond orbital (NBO) population analyses of 10 and the Ag(P4)2+ ion. Therefore, 1–3 are the first examples of species containing η2-coordinated tetrahedral P4 molecules.

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