The reaction of isolable dialkylstannylene 1 with an excess amount of CS2 produces an isomeric mixture of 3,3′-distanna-2,2′,4,4′-tetrathiabicyclobutylidene 8 and 3,7-distanna-2,4,6,8-tetrathiabicyclo[3.3.0]oct-1(5)-ene 9 with a ratio depending on the reaction conditions. Compounds 8 and 9 are separated by column chromatography and characterized by NMR spectroscopy and X-ray crystallography. Detailed investigation of the reaction has revealed that the initial product is 8, which isomerizes to 9 irreversibly under the catalytic influence of 1 as a Lewis acid. The above view is supported by the theoretical DFT calculations. Treatment of 1 with ArN═C═O [Ar = 2,6-iPr2C6H3] affords the corresponding carbamoyl(hydroxyl)stannane 11 via the hydrolysis of the corresponding silaaziridinone formed by the [1 + 2] cycloaddition reaction of 1 with the N═C double bond of the isocyanate. Stannylene 1 reacts with ArN═C═S, giving a mixture of complex products, while 1 does not react with CO2.

The reactions of isolable dialkysilylene 1 with 2-diazo-1,2-diphenylethanone and ethyl 2-diazo-2-phenylacetate gave elusive silacycles, 2H-1,2-oxasiletes 2 and 3, respectively, in high yields. Because these reactions occur at low temperatures of ca. −30 °C, initial complexation of the silylene to the carbonyl oxygen of the diazocarbonyl compounds is suggested to trigger dinitrogen elimination followed by cyclization. In contrast, a six-membered cyclic diazo compound 8 and 1-sila-2,3-diazabicyclo[3.3.0]oct-3-ene 10 were obtained in good yields by the reaction of 1 with less reactive ethyl 2-diazo-3-oxo-3-phenylpropanoate 7 and trimethylsilyldiazomethane 9. Molecular structures of 2, 3, 8 and 10 were determined by X-ray crystallography.

The structural diversity of bicyclo[1.1.0]tetrasilane has been analyzed using perturbation MO theory and DFT calculations. Among its four planar-cis (C2v), planar-trans (C2h), long-bond (C2v), and short-bond isomers (C2v) found theoretically, the first two are derived as the results of second-order Jahn–Teller distortion of the D2h isomer associated with the π–σ* orbital mixing. The last two are formed through orbital mixing in an isomer with a folded ring. The orbital mixing of the two σ*(Si–H) orbitals at the 1- and 3-silicon atoms into a π-type orbital between the atoms plays a crucial role in stabilizing the system by causing pyramidalization at the silicon atoms and by introducing the inverted σ-bond nature between the atoms. The π–σ* orbital mixing model is applied to predicting the structure of related compounds.

An isolable dicoordinate dialkylsilylene, 2,2,5,5-tetrakis(trimethylsilyl)silacyclopentane-1,1-diyl (6), was found to react with CO2 and ArN═C═X (X = O, S) smoothly to give the corresponding bis(silyl)carbonate, 4-imino-1,3-dioxasiletane and 4-imino-1,3-dithiasiletane derivatives in high yields, respectively. The molecular structures of these products were determined by X-ray crystallography. All these reactions are parallel to those of a hypercoordinate silylene with η5-pentamethylcyclopentadienyl ligands, decamethylsilicocene, reported by Jutzi et al. and are suggested to involve similarly the formation of the corresponding Si═X doubly bonded compounds (X = O, S) at the initial steps. Mechanistic details of the multistep reaction of a model dialkylsilylene with CO2 were investigated using DFT calculations.

The 1:2 reactions of isolable dialkylsilylene 1 with nitriles having electron-donating aromatic substituents gave 1,4-diaza-2-siloles with a hitherto-unknown type of ring system, in contrast to the previous studies showing exclusive formation of the corresponding 1,3-diaza-2-siloles; the reactions of 1 with aromatic nitriles bearing electron-withdrawing substituents afforded the latter ring system. The remarkable diversity of the reactions is explained by invoking the corresponding nitrile silaylides as key intermediates whose polarity switches depending on the substituents of nitriles.

The reactions of isolable dialkylsilylene 10 with various aldimines proceed smoothly at low temperatures to give diverse products depending on the substituents on the imine. The reactions of 10 with 4-XC6H4CHNPh [X = H (11a), MeO (11b), and Cl (11c)] give the corresponding silaaziridines 12a–12c in high yields, which are thermally very stable and remain intact in the air and moisture for a long time. In contrast, the reactions of 10 with 4-F3CC6H4CHNPh (11d) and 3,5-(F3C)2C6H3CHNPh (11e) having strong electron-withdrawing aryl substituents on imine carbon are accompanied by 1,2-trimethylsilyl migration rather unexpectedly to give silaazetidines 13d–13e incorporated into a bicyclo[3.2.0]heptane ring. The reaction of 10 with N-benzylbenzaldimine 11f affords the corresponding (dibenzylamino)silane 14f in a moderate yield. Molecular structures of 12a–12c, 13d–13e and 14f were determined by X-ray crystallography. All these reactions are proposed to occur via the initial formation of the corresponding imine silaylides, while the subsequent reactions leading to the final products are controlled by the electronic structure of the ylide depending on the substituents. N-Phenylbenzophenimine 11g does not react with 10.

Various reactions for a stable dialkylsilylene, 2,2,5,5-tetrakis(trimethylsilyl)silacyclopentane-1,1-diyl (1), are summarized and their mechanisms are discussed. Silylene 1 isomerizes to the corresponding silaethene via the 1,2-trimethylsilyl migration. Reduction of 1 with alkali metals affords the corresponding radical anion 1. −  with a relatively small 29Si hfs constant (2.99 mT) and a large g-factor (g = 2.0077) compared with those for trivalent silyl radicals. Photo-excitation of 1 generates the corresponding singlet excited state (11 ∗ ) with the lifetime of 80.5 ns. The excited state reacts with C=C double bond compounds including benzene, naphthalene, and (E)- and (Z)-2-butenes. Although the thermal reactions of 1 with haloalkanes occur via radical mechanisms, the insertion into O–H, Si–H and Si–Cl bonds proceeds concertedly via the three-membered cyclic transition states. The reaction of 1 with H2SiCl2 gives the Si–Cl insertion product exclusively, while the quantitative insertion to Si–H bond occurs when Me2SiHCl is used as a substrate. The origin of the rather unusual Si–H/Si–Cl selectivity is elucidated using DFT calculations. Silylene 1 adds to C=C, C≡C, and C=O π bonds to afford the corresponding silacycles as stable compounds. The importance of the carbonyl silaylides during the reactions of silylenes with aldehydes and ketones is emphasized.

Electronic and steric substituent effects on the insertion reactions of dimethylsilylene (1) and 2,2,5,5-tetrasilylsilacyclopentane-1,1-diyl (2′) into Si–H bonds are compared with those into Si–Cl bonds using DFT calculations at the B3LYP/6-31++G(d,p) level. For both 1 and 2′, the ΔG⧧ value for the insertion into a Si–H bond of SiH4 is close to and only ca. 1.5 kcal mol–1 lower than that into the Si–Cl bond of H3SiCl. Effects of in-plane substituents on the ΔG⧧ values for both Si–H and Si–Cl insertion reactions are mainly electronic and electron-withdrawing substituents lower the ΔG⧧ values. Sensitivity of the Si–H insertion to the substituent effects is similar to that of the Si–Cl insertion. Effects of out-of-plane substituents are largely steric, and their sensitivity for the Si–H insertion is close to that for the Si–Cl insertion. The ΔG⧧ values for the insertion reactions of sterically bulky 2′ are significantly larger than those of 1. The puzzling prior observation that a bulky isolable silylene inserts into the Si–Cl bond of H2SiCl2 and the Si–H bond of Me2SiHCl is explained on the basis of the substituent effects on the insertion reactions.

Three distortion modes, trans and cis bending and twisting, of ethylene, disilene, and digermene and their anions are discussed in terms of a generalized π–σ* orbital mixing model using PMO theory. The model predicts that trans- and cis-bent geometries of neutral dimetallenes may be stabilized from the planar geometry but with very small energy gains and that these geometries should be largely stabilized with significant bend angles and stabilization energies in dimetallene anions. Twisting around the double bonds of dimetallenes may lower the energy of the anions. The above prediction is verified by the potential energy surface calculations at the B3LYP/6-311++G(3df,3pd) level.

A series of dialkylsilylene-group 10 metal complexes (RH2Si)(Me3P)2M (M = Ni, Pd, Pt, RH2 = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) and novel (η6-arene)(dialkylsilylene)nickel complexes (η6-arene)(RH2Si)Ni were synthesized and characterized by NMR spectroscopy and X-ray analysis. The perpendicular geometry around the Si–M bond and the short Si–M distances of (RH2Si)(Me3P)2M complexes indicate the significant back donation from the metal to the silylene ligand.

A series of dialkylsilylene-group 10 metal complexes (RH2Si)(Me3P)2M (M = Ni, Pd, Pt, RH2 = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) and novel (η6-arene)(dialkylsilylene)nickel complexes (η6-arene)(RH2Si)Ni were synthesized and characterized by NMR spectroscopy and X-ray analysis. The perpendicular geometry around the Si–M bond and the short Si–M distances of (RH2Si)(Me3P)2M complexes indicate the significant back donation from the metal to the silylene ligand.

The reactions of isolable dialkysilylene 1 with 2-diazo-1,2-diphenylethanone and ethyl 2-diazo-2-phenylacetate gave elusive silacycles, 2H-1,2-oxasiletes 2 and 3, respectively, in high yields. Because these reactions occur at low temperatures of ca. −30 °C, initial complexation of the silylene to the carbonyl oxygen of the diazocarbonyl compounds is suggested to trigger dinitrogen elimination followed by cyclization. In contrast, a six-membered cyclic diazo compound 8 and 1-sila-2,3-diazabicyclo[3.3.0]oct-3-ene 10 were obtained in good yields by the reaction of 1 with less reactive ethyl 2-diazo-3-oxo-3-phenylpropanoate 7 and trimethylsilyldiazomethane 9. Molecular structures of 2, 3, 8 and 10 were determined by X-ray crystallography.

The 1:2 reactions of isolable dialkylsilylene 1 with nitriles having electron-donating aromatic substituents gave 1,4-diaza-2-siloles with a hitherto-unknown type of ring system, in contrast to the previous studies showing exclusive formation of the corresponding 1,3-diaza-2-siloles; the reactions of 1 with aromatic nitriles bearing electron-withdrawing substituents afforded the latter ring system. The remarkable diversity of the reactions is explained by invoking the corresponding nitrile silaylides as key intermediates whose polarity switches depending on the substituents of nitriles.

The reactions of isolable dialkylsilylene 10 with various aldimines proceed smoothly at low temperatures to give diverse products depending on the substituents on the imine. The reactions of 10 with 4-XC6H4CHNPh [X = H (11a), MeO (11b), and Cl (11c)] give the corresponding silaaziridines 12a–12c in high yields, which are thermally very stable and remain intact in the air and moisture for a long time. In contrast, the reactions of 10 with 4-F3CC6H4CHNPh (11d) and 3,5-(F3C)2C6H3CHNPh (11e) having strong electron-withdrawing aryl substituents on imine carbon are accompanied by 1,2-trimethylsilyl migration rather unexpectedly to give silaazetidines 13d–13e incorporated into a bicyclo[3.2.0]heptane ring. The reaction of 10 with N-benzylbenzaldimine 11f affords the corresponding (dibenzylamino)silane 14f in a moderate yield. Molecular structures of 12a–12c, 13d–13e and 14f were determined by X-ray crystallography. All these reactions are proposed to occur via the initial formation of the corresponding imine silaylides, while the subsequent reactions leading to the final products are controlled by the electronic structure of the ylide depending on the substituents. N-Phenylbenzophenimine 11g does not react with 10.