Experimental X-ray Electron Density Study of Atomic Charges, Oxidation States, and Inverted Ligand Field in Cu(CF₃)₄^–

Abstract

The electron density distribution of the complex monoanion Cu(CF 3 ) 4 in (Bu 4 N)[Cu(CF 3 ) 4 ] has been studied by high-resolution X-ray single-crystal diffraction and augmented with theoretical calculations. The study finds that the central copper bears an atomic charge of close to +1, while the occupancy of its d x 2 −y 2 orbital is only 1.26. Using topological analysis combined with theoretical calculations, the depopulation of d x 2 −y 2 is shown to be due to significant covalency in the Cu−C bonds. The combination of the monovalent picture and the covalency is interpreted as a confirmation of an inverted ligand field.

M−C Bond Homolysis in Coinage‐Metal [M(CF3)4]− Derivatives

“…Recent experimental spectroscopic measurements on [Cu(CF 3 ) 4 ] − salts also gave apparently conflicting results . Following X‐ray electron density studies, the estimated atomic charge on the copper atom is close to +1 . This estimate, however, by no means alters the oxidation state of the metal, which is assigned by pure convention .…”

“…Whereas the heaviest [Au(CF 3 ) 4 ] − derivative arises little suspicion, the oxidation state of the lightest [Cu(CF 3 ) 4 ] − derivative has been much debated . In fact, despite considerable effort to ascertain the bonding scheme of this organocopper derivative, this question still remains unanswered.…”

“…Averaging out the global charge ascribed to the donor system over the four ligands results in (CF 3 ) 0.5− donors, which interact with the metal centre in an almost ionic way (>80–95 %) . This bonding scheme has been confirmed recently, but with some emphasis put on the covalent character of the Cu−C bond, as already pointed out by Kaupp and von Schnering . Recent experimental spectroscopic measurements on [Cu(CF 3 ) 4 ] − salts also gave apparently conflicting results .…”

“…This estimate, however, by no means alters the oxidation state of the metal, which is assigned by pure convention . It has further been suggested that the [M(CF 3 ) 4 ] − anions would act as electrophilic trifluoromethylation agents, that is, as CF 3 + sources if they were true d 10 entities (Scheme b) . In order to shed some light on the nature of the M−C bonds in all three coinage‐metal derivatives [M(CF 3 ) 4 ] − (M=Cu, Ag, Au), we now report a comparative study of their decomposition pathways upon excitation using a combined experimental and theoretical approach.…”

“…Their substantially different stabilities must therefore be related to their electronic structure. We recently calculated the electronic structure of the silver complex 1 b , and the copper compound 1 a has itself been the object of intensive calculations . We now focused on the heaviest derivative 1 c and its relationship with its lighter homologues (Figure ).…”