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

Abstract

By using suitable synthetic procedures, we have first isolated the square-planar organosilver(III) compounds 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-[(CF 3 ) 2 AgX 2 ] À 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 À ligand affording rare five-coordinate Ag III species [(CF 3 ) 2 AgX 3 ] 2À . Interestingly, compound [PPh 4 ] 2 [(CF 3 ) 2 AgBr 3 ] (3) has been identified by SC-XRD methods as the first Ag III derivative with trigonal symmetry in general and trigonal bipyramidal geometry in particular. This unusual five-coordinate species also exhibits inverted ligand field.

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

“…For example, this exact case has been more simply described as stabilization (or destabilization) of the metal center through “inverted bonding”, 26 as the ligand field of the metal center itself is not inverted. 1 , 61 In metal complexes with such large amounts of metal–ligand covalency, referring to select few MOs with slightly larger Cu 3d admixtures as the Cu d-manifold is problematic, as the distribution of Cu character among the valence orbitals is vast and significantly lower than that which is seen in Cu(I) systems.…”

“…Comparatively, the calculated Loẅdin population of the unrestricted natural orbitals (UNOs) gave a Cu nd admixture that was much closer to the calculated spin population (Table S10). This is also true for the Cu(II) complex [Cu II (NHC 2 )] 2+ , which is calculated to have a Cu Loẅdin spin population of 61 canonical orbitals, which would qualify as an "inverted ligand field", despite experimental EPR evidence for a significantly large Cu hyperfine and thus Cu admixture into the ground state wave function. 32 In the case of the closed-shell Cu(I) and Cu(III) complexes, Cu nd admixtures in the HOMO and LUMO, respectively, according to the population analyses on the UNOs, are consistently much less than 10%, which is unrealistic in the case of the Cu(I) complex [Cu I (NHC 2 )] + considering the HOMO has dominant Cu 3d x 2 −y 2 character (Figure S24).…”

“…However, the suggestion that the lowering of the d-orbital character in the LUMO below 50% yields an inverted ligand field is not a unique suggestion, only a renaming, and the covalency of the system would remain equal for d orbital/ligand orbital displacements equidistant from the 50% mark. For example, this exact case has been more simply described as stabilization (or destabilization) of the metal center through “inverted bonding”, as the ligand field of the metal center itself is not inverted. , In metal complexes with such large amounts of metal–ligand covalency, referring to select few MOs with slightly larger Cu 3d admixtures as the Cu d-manifold is problematic, as the distribution of Cu character among the valence orbitals is vast and significantly lower than that which is seen in Cu­(I) systems.…”

Electrophilicity of neutral square-planar organosilver(iii) compounds

“…This is in contrast with the anionic halide systems [(CF 3 ) 2 AgX 2 ] − previously observed, where the association of an additional X − anion resulted in marked structural changes. 14 Regardless of the extent of distortion observed in each system, it can be concluded that the electrophilic character of the Ag(III) centre seems to be a general feature in SP-4 complexes. It would therefore be advisable to take it into account when trying to trace reliable reaction mechanisms, as this might lead to a better understanding of the chemical processes.…”

Cross‐Coupling Reactions Enabled by Well‐Defined Ag(III) Compounds: Main Focus on Aromatic Fluorination and Trifluoromethylation

“…Likewise, 40 was obtained by oxidation of its homoleptic Ag I derivative, [PPh 4 ][Ag I (CF 3 ) 2 ] ( 39 ) under local defect of PhICl 2 . Interestingly, in case of local excess of oxidant (X 2 ), the direct oxidative addition product [PPh 4 ][ trans‐ Ag III (CF 3 ) 2 X 2 ] (X=Cl, Br), was formed [53] . Opposed to the extremely aggressive behavior of AgF 3 or [AgF 4 ] − , [54] which are considered amongst the most powerful known oxidizers, 41 seems to be easier to handle, although it becomes easily hydrolysed to form [PPh 4 ][Ag III (CF 3 ) 3 (OH)], thus requiring the use of quartz vessel and its handling and stocking at low temperature.…”