Inverted Ligand Field Concept Extended to Gold Complexes

The Ligand Field theory (LF) for coordination compounds is based on the assumption that all the LCAO combinations of ligand lone pairs lie lower in energy than any suited metal atomic orbital (s, p or d). For the general acceptor character of the metal, coordination bonding is considered dative. An inverted situation (ILF) is instead found on combining a quite electropositive donor element (e.g., carbon, silicon and others) with one or more d orbitals of a particularly electronegative metal, e.g., a coinage one. Evidence emerged in particular from the low energy x2-y2 in the square planar complex [Cu(CF3)4]-, which pushed J.P Snyder to claim the Cu(I) vs. Cu(III) character, immediately opposed. Only recently, the point has been experimentally corroborated by XPS measurements and analyzed from the theoretical viewpoint. As shown in Figure 1, the b1g combination of the ligands lies higher than x2-y2, suggesting that the bond is not dative but due to a ? back-donation of metal electrons. Also, the LUMO indicates residual electrophilicity associated to the vacancy of the ligands, a point which has raised important discussion. As an extension of the ILF concept, the behavior of the heavier Au element, whose d orbitals lie >1eV lower than the Cu ones, will be addressed. Clearly, the retro-donation is, as in the Cu case, most pronounced for the known [Au(CF3)4]- complex. On the other hand, the Au(III) nature seems somewhat questionable also for other square planar complexes with less electronegative ligands, with the clear exception of F and O. The implicit electron redistribution affects the chemical reactivity of gold, which has remained peculiar for a long time. However, its recent revitalization has raised many intriguing questions, to which we will attempt some answer.