Doubly charged four-membered ring systems: Cyclobutadiene dianions and heavy cyclobutadiene dianions

Recent progress in the experimental chemistry of cyclobutadiene dianions (CBD(2-)s) containing a doubly charged four-membered ring system is reviewed. New families of silyl-substituted CBD(2-)s, cyclobutadienes (CBDs), and tetrahedranes are covered in detail. The aromaticity of the tetrasilyl- and diphenyldisilyl-CBD(2-)s have been studied on the basis of structural characteristics and magnetic properties. All of the CBD(2-)s show a strong diatropic ring current resulting from six pi-electron system. The preparation of CBD2- derivatives consisting of heavier group 14 elements (Si and Ge) is also included. They demonstrate a non-aromatic nature with a folded four-membered ring. 1 Introduction 2 Cyclobutadiene Dianions (CBD(2-)s) 2.1 Preparation of CBD Complexes 2.2 Transmetalation of CBD Complexes 2.3 Tetrasilyl-CBD2-S 2.4 Diphenyl-Disityl-CBD2-S 3 Cyclobutadienes (CBDs) 4 Tetrahedranes 5 Heavy Cyclobutadiene Dianions (CBD2-S) 6 Conclusion.