In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

https://pubs.acs.org/doi/pdf/10.1021/cr900294f

Stable Heavier Carbene Analogues

Recent topics in the chemistry of heavier congeners of carbenes

http://www.rsc.org/suppdata/CC/b9/b901940e/b901940e.pdf

Bicyclic-guanidines, -guanidinates and -guanidinium salts: wide ranging applications from a simple family of molecules

Some recent advances in the chemistry of silicon and its homologues in low coordination states

https://www.aoc.uni-wuppertal.de/fileadmin/Chemie/Anorganische_Chemie/Prof_Mohr/Skripte/Gr14_Alkinanaloga.pdf

Silicon, germanium, tin and lead analogues of acetylenes

Synthesis and Crystal Structures of the Compounds [Sn{C(SiMe2Ph)3}Cl]2, [Pb{C(SiMe3)3}Cl]3, and [M{C(SiMe3)2(SiMe2OMe)}Cl]2 (M = Sn or Pb)

A Novel Organolead(II) Species the Plumbacycloalkane Derivative [CH2SiMe2C(SiMe3)2PbC(SiMe3)2SiMe2Ch2]

The title compound, thought to be the first monoorganolead(II) compound, has been prepared and structurally characterized; the organic ligands are in a tran-disposition and the lone-pair on lead is stereochemically active.

The first monoorganolead(II) derivative, [PbR(-Cl)}2] where R = C(SiMe2Ph)3

An ether-free, internally coordinated dialkylcalcium(II) complex

An Unusual Alkyltrihydroaluminate. Crystal Structures of [AlH2{C(SiMe3)2(SiMe2OMe)}(μ-H)Li(THF)], [AlCl2{C(SiMe2OMe)(SiMe3)2(THF)], and [Li{C(SiMe2OMe)(SiMe3)2}(THF)2] (THF = Tetrahydrofuran)

Sebnem E. Sözerli

Papers

Synthesis and Crystal Structures of the Compounds [Sn{C(SiMe2Ph)3}Cl]2, [Pb{C(SiMe3)3}Cl]3, and [M{C(SiMe3)2(SiMe2OMe)}Cl]2 (M = Sn or Pb)

A Novel Organolead(II) Species the Plumbacycloalkane Derivative [CH2SiMe2C(SiMe3)2PbC(SiMe3)2SiMe2Ch2]

The first monoorganolead(II) derivative, [PbR(-Cl)}2] where R = C(SiMe2Ph)3

An ether-free, internally coordinated dialkylcalcium(II) complex

An Unusual Alkyltrihydroaluminate. Crystal Structures of [AlH2{C(SiMe3)2(SiMe2OMe)}(μ-H)Li(THF)], [AlCl2{C(SiMe2OMe)(SiMe3)2(THF)], and [Li{C(SiMe2OMe)(SiMe3)2}(THF)2] (THF = Tetrahydrofuran)