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

Reactions of hydrosilanes with transition metal complexes and characterization of the products.

The transition-metal chemistry of amidinatosilylenes, -germylenes and -stannylenes

This third review in a series involving reactions of hydrosilanes and transition metal complexes and characterization of the products covers the period 2009 thru 2013. After a brief discussion of other synthetic methods used for the formation of Si-TM complexes, Section 3 provides an extended discussion of the types of ligands and metal complexes used as reactants with hydrosilanes. The increase in use of pincer ligands in forming stable, isolable complexes is featured. Three tables list the complexes reported for primary, secondary, and tertiary silanes. Many reactions leading to SiTM complexes are initiated by loss of a ligand prior to oxidative addition of the hydrosilane or by a metathesis reaction. Structural data are tabulated for the isolated complexes and provide the Si-TM bond ranges for the elements in the “d” block. A major section on nonclassical (σ or agostic) complexes includes two general groupings with Si–H···TM and M–H···Si interactions. A section on solution processes identified by NMR s...

Reactions of Hydrosilanes with Transition Metal Complexes.

Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon via Si—H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η3-H2SiRR' complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers, and the importance of these catalysts to the development of new hydrosilation reactions is discussed.

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Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

Stable transition-metal complexes with divalent silicon ligands (silylene complexes) have been extensively studied because of their important role in many catalytic processes. Since the pioneering works by Zybill et al. and Tilley et al. , various base-stabilized and base-free silylene complexes have been synthesized and their versatile reactivity has been well explored. Although complexes with two or more silylene ligands are expected to show interesting bonding properties and reactivities that are not observed in monosilylene complexes, such complexes are still limited to donorbridged bis(silylene) complexes and complexes having cyclic diaminosilylenes as ligands. During the course of our study on the application of dialkylsilylene 1, which is the

Fourteen-electron bis(dialkylsilylene)palladium and twelve-electron bis(dialkylsilyl)palladium complexes.

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.

Synthesis and structures of (dialkylsilylene)bis(phosphine)-nickel, palladium, and platinum complexes and (η6-arene)(dialkylsilylene)nickel complexes

A novel dinuclear palladium complex with one bridging dialkylsilylene ligand 6 was synthesized by the reaction of isolable dialkylsilylene 7 (R2Si:) with two equiv of bis(tricyclohexyl)phosphinepal...

Chieko Watanabe

Papers

Fourteen-electron bis(dialkylsilylene)palladium and twelve-electron bis(dialkylsilyl)palladium complexes.

Synthesis and structures of (dialkylsilylene)bis(phosphine)-nickel, palladium, and platinum complexes and (η6-arene)(dialkylsilylene)nickel complexes

Dinuclear palladium complex with a bridging dialkylsilylene ligand