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

The Chemistry of Aluminum(I), Silicon(II), and Germanium(II)

The First Isolable Dialkylsilylene

Boron cations are elusive and highly electrophilic species that play a key role in the chemistry of boron. Despite early interest in the chemistry of boron cations, until recently they have largely remained chemical curiosities. However, hints at harnessing their potential as potent electrophiles have begun to appear and developments in weakly coordinating anion technology suggest that this is an area of research that is ripe for exploration. It has been nearly 20 years since the last major review on boron cations; herein we summarize the progress in the area since that time.

Borinium, borenium, and boronium ions : Synthesis, reactivity, and applications

Activation of the Si-B interelement bond: mechanism, catalysis, and synthesis.

Reaction of dichlorobis(pentamethylcyclopentadienyl)silane with naphthalene-lithium, -sodium, or -potassium leads to elemental silicon and to decamethylsilicocene, (Me5C5)2Si (4). Compound 4 is formed as the only product in the reduction of dibromobis(pentamethylcyclopentadineyl)silane with anthracene-potassium. 4 is a thermally stable, colorless, air-sensitive π-complex. Its NMR spectra are typical for a group 14 metallocene; the 29Si-NMR signal appears at very high field strength (δ = −398 ppm). CV and MS data of 4 prove the instability of the (Me5C5)2Si⊖ ion, which easily looses a Me5C5 radical. X-ray crystallographic studies show the presence of two geometrical isomers, 4a and 4b, in the monoclinic unit cell. Isomer 4a is isotypical with decamethylferocene, isomer 4b possesses the expected bent-metallocene-type structure. Space-filling models indicate the interplane angle in 4b to be of the largest possible value. Due to GED studies, 4 has a bent-metallocene-type structure in the gas phase. The He(I) PE spectrum of 4 is compared with those of the heavier homologues (Me5C5)2Ge (6), (Me5C5)2Sn (7), and (Me5C5)2Pb (8). A strong shift to higher energy of the band assigned to the group 14 element lone pair is observed in going from 8, 7, or 6 to 4. Calculations have been carried out for structural models of the parent silicocene, (H5C5)2Si, on the basis of the MNDO procedure and of the HF theory (STO-3G and STO-3G* basis sets). The calculated geometrical parameters and orbital energies are compared with those derived from the GED experiment and from the PE spectrum. The bathochromic shift in the electronic absorption spectra going from 4 to 6–8 is explained on the basis of MNDO calculations for the parent metallocenes.



Decamethylsilicocen - die erste stabile Silicium(II)-Verbindung: Synthese, Struktur und Bindungsverhaltnisse



Die Reaktion von Dichlorbis(pentamethylcyclopentadienyl)silan mit Naphthalin-Lithium, -Natrium oder -Kalium fuhrt zu elementarem Silicium und zu Decamethylsilicocen (4). Bei der Reaktion von Dibrombis(pentamethylcyclopentadienyl)silan mit Anthracen-Kalium wird ausschlieslich 4 gebildet. 4 ist ein thermisch stabiler, farbloser, luftempfindlicher π-Komplex. Seine NMR-Spektren sind typisch fur Metallocene der Gruppe 14; das 29Si-NMR-Signal erscheint bei sehr hohen Feldstarken (δ = -398 ppm). CV- und MS-Daten von 4 belegen die Instabilitat des (Me5C5)2Si⊖-Ions, das leicht ein Me5C5-Radikal verliert Nach Rontgenstrukturuntersuchungen liegen zwei geometrische Isomere, 4a und 4b, in der monoklinen Elementarzelle vor. Isomer 4a ist isotyp mit Decamethylferrocen, Isomer 4b besitzt die erwartete Struktur eines gewinkelten Metallocens. Nach Modellbetrachtungen liegt in 4b der grostmogliche Interplanar-Winkel vor. GED-Studien belegen fur 4 eine gewinkelte Metallocen-Struktur in der Gasphase. Das He(I)-PE-Spektrum von 4 wird mit denjenigen der schwereren Homologen (Me5C5)2Ge (6), (Me5C5)2Sn (7) und (Me5C5)2Pb (8) verglichen. Fur die Bande, die dem freien Elektronenpaar des Gruppe-14-Elements zugeordnet wird, ist beim Ubergang von 8, 7 oder 6 zu 4 eine starke Verschiebung zu hoherer Energie zu beobachten. Strukturmodelle der Stammverbindung (H5C5)2Si sind auf der Basis des MNDO-Konzepts und der HF-Theorie (STO-3G- und STO-3G*-Basissatze) berechnet worden. Die berechneten geometrischen Parameter und Orbital-Energien werden mit denjenigen aus dem GED-Experiment und dem PE-Spektrum verglichen. Die bathochrome Verschiebung in den Elektronenanregungsspektren beim Ubergang von 4 zu 6–8 wird unter Zugrundelegung von MNDO-Rechnungen fur die H-substituierten Metallocene erklart.

Decamethylsilicocene — The first stable silicon(II) compound: Synthesis, structure, and bonding

Reaction of Decamethylsilicocene with Group 13 Element Halides: Insertions, Rearrangements, and Eliminations

Chemistry of decamethylsilicocene: Oxidative addition of compounds with XH bonds (X F, Cl, Br, O, S)

In general, decamethylsilicocene (1) reacts with Bronsted acids under oxidative addition yielding SiIV compounds of the type (Me5C5)2Si(H)X (2)–(7); as an exception, the cyclotetrasilane (Me5C5SiF)4(8) is formed in the reaction with tetrafluoroboric acid and has been characterized by an X-ray diffraction study.

Udo Holtmann

Papers

Decamethylsilicocene — The first stable silicon(II) compound: Synthesis, structure, and bonding

Reaction of Decamethylsilicocene with Group 13 Element Halides: Insertions, Rearrangements, and Eliminations

Chemistry of decamethylsilicocene: Oxidative addition of compounds with XH bonds (X F, Cl, Br, O, S)

Protonation of decamethylsilicocene [bis(pentamethylcyclopentadienyl)silicon]

Chemistry of Decamethylsilicocene: Oxidative Addition of Compounds with X-H Bonds (X: F, Cl, Br, O, S).