The Synthesis of Stable, Isolable Planar-Tetracoordinate Carbon Compounds

In situ generated (η²-alkyne)metallocene complexes 16 can be trapped with a variety of Lewis acidic main-group organometallics as scavengers forming compounds CP₂M¹(μ-η¹: η²-R¹C¹C²R²) (μ-X)M²R³ ₂ 14 (M¹ = Zr, Hf; M² = Al, Ga, B). The highly reactive (alkyne)metallocene complexes 16 are generated by reaction of either dimethylmetallocene compounds 17 or oligomeric [Cp₂Zr(H)CI] 21 with group 3 metal alkynyl compounds 18. A further possibility for the synthesis of the intermediates 16 is the reaction of phosphane stabilized (alkyne)metallocene complexes 20 with Lewis acids forming a Lewis acid-phosphane adduct and the unsaturated (alkyne)metallocene species 16. Compounds 14 produced from 16 by addition of Lewis acidic organometallics contain as a remarkable feature a carbon atom (C²) that shows four strong bonding interactions to neighbouring atoms all lying perfectly in one plane. Several of such unusual planar-tetracoordinate carbon compounds were synthesized and characterized. X-ray diffraction of a number of these compounds 14 has proven the unusual geometry at the square-planar carbon center C². 1. Introduction 2. Theoretical Considerations 3. Attempts to Synthesize Stable Square-Planar All-Carbon Systems 4. Organometallic Compounds Containing Planar-Tetracoordinate Carbon 5. Conclusions