Double-bonded species

Compounds with Pn-Pn (Pn = Sb, Bi) Multiple Bonds 15.2.4.1 Double-Bonded Species 

The dianion of 37 features a Bi=Bi bond distance (2.838(1) A) comparable to that observed 

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Cyclic Si-, Ge-, Sn- and Pb-analogs of carbenes and alkenes 99EJI373. Germylenes and germanium double-bonded species in heterocyclic organoger-manium chemistry 99KGS1155. 

The ene reaction as a reaction principle has been first recognized and systematically investigated by Alder It is a thermal addition reaction of a double bond species 2—the enophile—and an alkene 1—the ene—that has at least one allylic hydrogen. The intramolecular variant is of greater synthetic importance than is the intermolecular reaction. 

In the previous reviews,1 la,d f syntheses of many examples of transient silicon-oxygen double bond compounds such as MeHSi=0, Me2Si=0, H2Si=0 (2), (H0)HSi=0 (silanoic acid), and (H0)2Si=0 (silicic acid) have been described, and they are reportedly isolated as stable species in the low temperature matrices. However, the stabilization of this extremely reactive double bond species is very difficult, and no stable example of silanone (RR/Si=0) has been isolated until now even by the methods of thermodynamic or kinetic stabilization. 

A variety of preparation methods are known for transient germanium-chalcogen double bond species some of them seemed to be useful also for the synthesis of kinetically stabilized systems. Indeed, the reaction of a germylene with an appropriate chalcogen source was found to be one of the most versatile and general methods for the synthesis of stable germanium-containing heavy ketones (Scheme 21). 

This interpretation of the experimental results was corroborated by ab initio calculations on the relative stabilities of a series of double bond compounds, [H2Pb=X] and their plumbylene type isomers, [frans-H-Pb-X-H] and [ciT-H-Pb-X-H] (X = O, S, Se, and Te) (vide supra) 6 The results of the theoretical calculations and experiments for lead-chalcogen double bond species are in sharp contrast to those of the other Group 14 element analogues which do not isomerize to the divalent compounds.14b... 

Three-membered rings are intimately linked with divalent and double-bonded species and are discussed separately below. 

Double-bonded species (i.e., dicoordinate ) , cr2 metal species) were reported for As, Sb, and Bi.2 The stability of these species toward the formation of oligomers with metal-carbon single bonds is increased by steric protection with... 

An extensive range of compounds with homonuclear or heteronuclear single or double bonds between As, Sb, or Bi is known. They include the simple tetraorganodimetallic species, R2MMR2, double bonded species RM=MR, linear tri- and tetra-metallated species, and mono- or polycyclic compounds. In addition, adducts with coordinative metal-metal bonds are known.44-46 463 Examples for cationic species with Sb-Sb bonds are Me2SbSbMe2+ and... 

Other group 14 carbon double bonded species. 909... 

Irradiation of the (r -CsH5)2V2 (C0)5, V=V double bonded species results in loss of CO, not V=V bond cleavage upon photoexcitation (54). The crucial result comes from an experiment where the (n-CsHs)2V2(CO)5 is irradiated at -50°C in the presence of PEt2Ph in THF solution. The only product observed is (n-C5H5)2V2(CO)1(PEt2Ph. 

In the past few decades, almost all of the heavier chalcogen analogues of ketones, i.e. thioketones, selenoketones, and teUuroketones, have been synthesized and characterized. Both thermodynamic and kinetic stabilization methods have been applied to stabilize these unstable double-bond species. In contrast to the doubly bonded systems between carbon and heavier chalcogens, heavier group 14 element analogues of ketones are much more reactive and unstable and hence their structures and properties have not been fully disclosed until recently. ... 

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

Notable advances in the last decade are isolation of kinetically stabilized silicon-containing donble-bonded species. As we have seen already, a variety of sterically bulky substituents have been utilized to stabihze silicon-centered reactive intermediates. This kinetic stabilization is the key issue also to isolate silicon double-bonded species. For example, the synthesis of isolable silicon analog of benzene and other aromatic compounds has been a target for a long time. One example is shown below for the generation of transient silabenezene. ... 

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