The Fluoride Method: Access to Silver(III) NHC Complexes.

We have synthesized the first silver(III) carbene complexes, (CF3 )3 Ag(NHC), by direct reaction of the silver(III) fluoride precursor complex [PPh4 ][(CF3 )3 AgF] with different imidazolium salts. This novel methodology circumvents the use of free NHC molecules. The silver(III) carbene complexes thus prepared are unprecedented and show remarkable thermal stabilities. They display square-planar or square-pyramidal geometry. Following our calculations, the electronic structure of a model representative complex exhibits Inverse Ligand Field (ILF). The compounds reported herein are synthetic analogues of the elusive difluorocarbene and carbonyl species proposed as intermediates in the acidic decomposition of [Ag(CF3 )4 ]- . The synthetic procedure reported is envisaged to enable access to carbene complexes of other late transition-metals in high oxidation states.

Stability of AgIII towards Halides in Organosilver(III) Complexes.

The involvement of silver in two-electron AgI /AgIII processes is currently emerging. However, the range of stability of the required and uncommon AgIII species is virtually unknown. Here, the stability of AgIII towards the whole set of halide ligands in the organosilver(III) complex frame [(CF3 )3 AgX]- (X=F, Cl, Br, I, At) is theoretically analyzed. The results obtained depend on a single factor: the nature of X. Even the softest and least electronegative halides (I and At) are found to form reasonably stable AgIII -X bonds. Our estimates were confirmed by experiment. The whole series of nonradiative halide complexes [PPh4 ][(CF3 )3 AgX] (X=F, Cl, Br, I) has been experimentally prepared and all its constituents have been isolated in pure form. The pseudohalides [PPh4 ][(CF3 )3 AgCN] and [PPh4 ][(CF3 )3 Ag(N3 )] have also been isolated, the latter being the first silver(III) azido complex. Except for the iodo compound, all the crystal and molecular structures have been established by single-crystal X-ray diffraction methods. The decomposition paths of the [(CF3 )3 AgX]- entities at the unimolecular level have been examined in the gas phase by multistage mass spectrometry (MSn ). The experimental detection of the two series of mixed complexes [CF3 AgX]- and [FAgX]- arising from the corresponding parent species [(CF3 )3 AgX]- demonstrate that the Ag-X bond is particularly robust. Our experimental observations are rationalized with the aid of theoretical methods. Smooth variation with the electronegativity of X is also observed in the thermolyses of bulk samples. The thermal stability in the solid state gradually decreases from X=F (145 °C, dec.) to X=I (78 °C, dec.) The experimentally established compatibility of AgIII with the heaviest halides is of particular relevance to silver-mediated or silver-catalyzed processes.

A Five-Coordinate Compound with Inverted Ligand Field: An Unprecedented Geometry for Silver(III).

By using suitable synthetic procedures, we have first isolated the square-planar organosilver(III) compounds [PPh4 ][trans-(CF3 )2 AgX2 ] [X=Cl (1 a), Br (2 a)]. The geometry and stereochemistry of the chloro-derivative 1 a have been unambiguously established by single-crystal X-ray diffraction (SC-XRD) methods. Following our calculations on the relative stability of the cis-/trans-[(CF3 )2 AgX2 ]- couples (X=F, Cl, Br, I), the experimentally obtained compounds 1 a and 2 a appear to be kinetically favored stereoisomers. They display some tendency to associate an additional X- ligand affording rare five-coordinate AgIII species [(CF3 )2 AgX3 ]2- . Interestingly, compound [PPh4 ]2 [(CF3 )2 AgBr3 ] (3) has been identified by SC-XRD methods as the first AgIII derivative with trigonal symmetry in general and trigonal bipyramidal geometry in particular. This unusual five-coordinate species also exhibits inverted ligand field.

Homoleptic Trifluoromethyl Derivatives of AgI and AgIII.

The homoleptic silver(I) compound [PPh4 ][CF3 AgCF3 ] (1) provides a convenient entry to the homoleptic silver(III) derivative [PPh4 ][Ag(CF3 )4 ] (2). Once isolated as pure substances, these compounds exhibit marked thermal stabilities. Their structural and spectroscopic properties have been experimentally established. Moreover, their electronic structures have been calculated by theoretical methods. The electronic structure of the oxidized species [Ag(CF3 )4 ]- provides a new case of ligand-field inversion caused by the CF3 ligands.