Swimming against the Stream? A Discussion of the Bonding in d6 and d8 Fluoro Complexes and Its Consequences for Catalytic Applications

Abstract

Fluoride has recently found increasing application as coligand both in complexes of late transition metals and as a reagent in asymmetric catalysis. In recent papers, this topic was reviewed, with emphasis on the role played by so-called ‘push-pull interactions' between the fluoro ligand and a π acid in the stabilization of fluoro complexes with a d6 or d8 electron configuration. This picture has led to the concept that fluoride is the strongest π-donor in the halide series. Herein, the latter concept is discussed and criticized. In particular, the effect of the ionicity of the metal-fluoride bond is proposed as an alternative explanation to most of the observations that show a ‘reversed' halide order. The ionic character of the MF bond also accounts for its intrinsic reactivity and suggests some strategies for the stabilizatioin of fluoro complexes, including the use of coordinative unsaturation. The scope and limitations of the application of d6 and d8 fluoro complexes in catalysis are also discussed. The most promising application is the use of 16-electron fluoro complexes in the metal-promoted formation of the CF bond.