This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

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

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H2, NH3 and P4) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes.

Stable singlet carbenes as mimics for transition metal centers

still today the most severely bent acyclic allene known, with a C=C=C bond angle of 170.18. They demonstrated that the nonlinearity was due to packing effects in the crystal. To induce greater bending in an allene it is necessary to constrain the C=C=C p system into a ring, but this eventually leads to very unstable compounds. So far, the diphosphoruscontaining six-membered ring A3 is the most severely bent allene that has been isolated and characterized by crystallography; it has a C=C=C angle of 155.88. In marked contrast with all-carbon allene fragments (C= C=C), crystallographic and computational studies of allenes that are based on heavier Group 14 elements (E=E=E where E= Si, Ge; B1 and B2, respectively) demonstrate that they are highly flexibile, and exhibit a bent structure (136.58 and 122.68, respectively). The striking differences between the geometry between acyclic all-carbon allenes A1 and their heavier element congeners B1 and B2 is mainly due to the “first long-row anomaly”, as described byGr7tzmacher. The first long-row elements tend to form hybrids from s and p orbitals that lead to the familiar linear, trigonal, and tetragonal bonding geometries of the carbon compounds. Second long-row elements largely avoid hybridization. Among the consequences is that second and higher row elements are generally reluctant to form multiple bonds, and therefore heavier element–heavier element p bonds are weak. From this analysis, we reasoned that weakening the p bonds of all-carbon allenes A1 should make them more flexible, and ultimately lead to a bending of the otherwise rigid and linear C=C=C skeleton. It has been demonstrated that the carbon–carbon p bond of alkenes can be weakened by inducing either a diradical or zwitterionic character, as evidenced in each case by the twisting from planarity and lengthening of the C=C bond. The former option can be effected by steric congestion, as in the symmetrical compound C1 (twist angle= 668 ; dC=C = 1.39 :), whereas the latter is a result of p-bond polarization, as in C2 (twist angle= 868 ; dC=C = 1.47 :). [12] The steric approach cannot be extended to allenes because the termini are too remote for significant interactions. Therefore, the only possible way to weaken the p bonds of allenes is by polarization, which can be accomplished by either a push–pull or a push–push substitution pattern. Push–pull allenesD have a carbene character as inD’ (& = empty orbital), and they are prone to dimerization. Consequently, the best choice for preparing bent allenes is a push–push substitution pattern as in E, which should promote the dicarbanionic resonance form E’. Herein we report the isolation of an acyclic allene featuring a C=C=C bond angle of 134.88. Importantly, we show that bent allenes should not be considered as laboratory curiosities, but instead a novel class of strong h-donor ligands for transition-metal centers. [*] Dr. C. A. Dyker, V. Lavallo, B. Donnadieu, Prof. G. Bertrand UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957) Department of Chemistry, University of California Riverside, CA 92521-0403 (USA) Fax: (+1)951-827-2725 E-mail: guy.bertrand@ucr.edu Homepage: http://research.chem.ucr.edu/groups/bertrand/guybertrandwebpage/

Synthesis of an Extremely Bent Acyclic Allene (A “Carbodicarbene”): A Strong Donor Ligand

A Brief Survey of Our Contribution to Stable Carbene Chemistry

Trigermaallene and 1,3-Digermasilaallene

The reactions of stable cyclic dialkylgermylene 2 and dialkylstannylene 3 with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical (2 equiv) gave the corresponding 1:2 adducts 4 and 5, respectively, which were characterized by NMR, MS, and X-ray analyses. The kinetics of the stepwise addition of two TEMPO molecules to germylene 2 revealed that the initial addition of TEMPO to 2 was 1010 times slower than the second TEMPO addition to the resulting germyl radical. The origin of the rate difference was discussed on the basis of the qualitative perturbation theory. In contrast to the reactions of 2 and 3, the reaction of dialkylsilylene 1 with TEMPO gave an interesting 1,3-dioxadisiletane derivative.

Addition of stable nitroxide radical to stable divalent compounds of heavier group 14 elements.

The synthesis, X-ray and spectroscopic analysis, and unusual bonding and structure among heavy group-14 element congeners of allene (heavy allenes) [R2M═M′═MR2; M = M′ = Si (1a), M = Si, M′ = Ge (1...

Hidenori Masuda

Papers

Trigermaallene and 1,3-Digermasilaallene

Addition of stable nitroxide radical to stable divalent compounds of heavier group 14 elements.

Unusual bonding in trisilaallene and related heavy allenes.

Diverse reactions of nitroxide-radical adducts of silylene, germylene, and stannylene

Synthesis, crystal structure refinement and electrical properties of uranium oxysulfide, UOS