The reaction of (Me3Si) and it is only by considering@] metal-metal bonding between seemingly closed-shell Au(1) atoms, based largely on relativistic effects ["], that this phenomenon has been rationalized in very recent theoretical treatments.Quaternary phosphoniurn salts of the type { [(R3P)-Au],P}+ X-, for which a square-pyramidal structure has also been predicted in these theoretical cannot be isolated with standard tertiary phosphanes R3P, except for systems with the bulky ligand (tBu),P, where at least NMR evidence has been obtained["] for a cation {[(tBu)~PAu],P} +. Chemical shifts and doublet/quintet multiplicities in the 31P-NMR spectra are proof of the cation stoichiometry, but are inconclusive regarding the geometry at the central phosphorus atom ["] (tetrahedral vs. squareplanar). We have now been able to isolate the BF, and BPb-salts of this cation and to convert it into an unusual boroxinate salt, the crystal structure of which has been determined.
Synthesis and Properties of the Phosphonium SaltsTreatment of tris(trimethylsilyl)phosphane['4 with tris[(tritert-butylphosphane)gold(I)]oxonium tetrafluor~borate"~' in tetrahydrofuran at -78 "C with protection against directly incandescent light leads to a reaction mixture, from which a series of products can be precipitated by the addition of increasing quantities of pentane['1.'41. The tetrakis-[(tri-tert-butylphosphane)gold(I)]phosphonium tetrafluoroborate salt (la) appears as the last fraction and is obtained as an oily product, which is stable against air and water. Its NMR parameters have been confirmed ["], but attempts to crystallize the material have failed. The precipitates appearing upon addition of the first quantities of pentane to the reaction mixture are higher nuclearity clusters, the indentity of which is still under investigation. Penta-and hexanuclear compounds have been detected by analytical and spectroscopic techniques"'].In an attempt to convert l a into the tetraphenylborate salt l b , which was hoped to crystallize more readily, the tetrafluoroborate salt was treated with Na[BPh4] in acetonitrile. A series of extraction and crystallization steps (Experimental) lead to a pure product, for which cation and anion FAB mass spectrometry has confirmed the presence of the tetragoldphosphonium cation { [(~Bu)~PAu]~P) + and the BPh, anion beyond any doubt. Again, however, the material could not be crystallized.The NMR spectra of 1 b are virtually identical with those of la, except for the resonances of the BPh, anion replacing those of the BF, anion. This result indicates that both salts are dissociated in polar solvents to give the same cation.This cation exhibits only one set of NMR signals ('H, 13C, 31P) for four equivalent [(tBu),PAu] units attached to a central phosphonium center. The resonance of the central phosphorus atom appears as a 1:4:6:4:1 quintet, again indicating the equivalence of the four (phosphane)gold ligands. As mentioned in the introductory remarks, these multiplicity data do not allow a distinction to be made betwe...