New bonding modes of carbon and heavier group 14 atoms Si–Pb
Recent theoretical studies are reviewed which show that the naked group 14 atoms E = C–Pb in the singlet 1D state behave as bidentate Lewis acids that strongly bind two σ donor ligands L in the donor–acceptor complexes L→E←L. Tetrylones EL2 are divalent E(0) compounds which possess two lone pairs at E. The unique electronic structure of tetrylones (carbones, silylones, germylones, stannylones, plumbylones) clearly distinguishes them from tetrylenes ER2 (carbenes, silylenes, germylenes, stannylenes, plumbylenes) which have electron-sharing bonds R–E–R and only one lone pair at atom E. The different electronic structures of tetrylones and tetrylenes are revealed by charge- and energy decomposition analyses and they become obvious experimentally by a distinctively different chemical reactivity. The unusual structures and chemical behaviour of tetrylones EL2 can be understood in terms of the donor–acceptor interactions L→E←L. Tetrylones are potential donor ligands in main group compounds and transition metal complexes which are experimentally not yet known. The review also introduces theoretical studies of transition metal complexes [TM]–E which carry naked tetrele atoms E = C–Sn as ligands. The bonding analyses suggest that the group-14 atoms bind in the 3P reference state to the transition metal in a combination of σ and π∥ electron-sharing bonds TM–E and π⊥ backdonation TM→E. The unique bonding situation of the tetrele complexes [TM]–E makes them suitable ligands in adducts with Lewis acids. Theoretical studies of [TM]–E→W(CO)5 predict that such species may becomes synthesized.