The heavier group 14 carbenes are silylenes, R2Si:, germylenes R2Ge: (example diphosphagermylene), stannylenes R2Sn: and plumbylenes R2Pb:, collectively known as metallylenes and regarded as monomers for polymetallanes.
[1] The oxidation state for these compounds is +2 and stability increases with principal quantum number (moving down a row in the periodic table).
Group 14 carbene analogs do not form hybrid orbitals but instead retain (ns)2(np)2 electron configuration due to the increasing s p gap for larger elements.
General methods for the synthesis of carbon-substituted (aryl or alkyl) metallylenes are reduction of M4+ species or substitution reactions at M2+ halides.
The first isolable dialkylgermylene was synthesised in 1991:[2] Stable diarylgermylenes also require bulky ligands:[3] The C-M-C bond angle in metallylenes is less than 120° confirming hybridization other than sp2.