Concept
Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16. In group 13 elements the boron carbene analog is called a borylene or boranylidene. 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. The oxidation state for these compounds is +2 and stability increases with principal quantum number (moving down a row in the periodic table). This makes dichloroplumbylene PbCl2 and dichlorostannylene SnCl2 stable ionic compounds although they exist as polymers or ion pairs. 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. Two electrons remain in an s-orbital and therefore their compounds have exclusively singlet ground states and not the triplet ground state which can be observed in carbenes depending on the substituents. The s-orbital (lone pair) is inert and the vacant p-orbital is very reactive. Stable group 14 carbenes require stabilization of this p-orbital which is usually accomplished by coordination of a Cp* ligand or coordination to nitrogen, oxygen or phosphorus containing ligands, although stabilization can be achieved through steric protection alone. General methods for the synthesis of carbon-substituted (aryl or alkyl) metallylenes are reduction of M4+ species or substitution reactions at M2+ halides. Stable metallylenes require bulky substituents in order to prevent nucleophilic attack of the metal center at the p-orbital.