Stannenes Distannenes

In contrast to the extensively studied doubly bonded systems of silicon and germanium, the chemistry of the corresponding tin compounds, i.e. distannenes and stannenes, has not been fully disclosed yet probably due to the much higher reactivity and instability of such low-coordinated organotin compounds. In the following sections we briefly describe the synthesis and properties of stable distannenes and stannenes. 

The suffix a can be added to the stem of the substituent (giving stanna) and used to indicate replacement of carbon. This is most useful with cyclic compounds, thus cyclo-(CH2)5SnMe2 is 1,1-dimethylstannacyclohexane. Doubly bonded compounds are similarly named as alkenes with one or two of the doubly-bonded atoms replaced by tin the compound R2Sn=CR2 is a stannene, and iUSrwSnfU is a distannene. 

Recent years have seen important developments in the chemistry of tin(II) compounds and compounds with multiple bonds to tin (Scheme 1-3). The cyclopentadi-enyltin(II) compounds, which are formed from CpM and SnCL, are pentahapto monomers. When R is a simple alkyl or aryl group, the stannylenes R2Sn(II) are known only as short-lived reactive intermediates, but when the organic group is bulky [e.g. bis(trimethylsilyl)methyl or 2,4,6-trisubstituted aryl], as indicated by R in Scheme 1-3, the monomeric stannylenes, R 2Sn , have been isolated, and have provided routes to the stannenes (R 2Sn=CR.2) and distannenes (R 2Sn=SnR 2), and other compounds with a multiple bond to tin. 

Alkylation or arylation of stannous chloride gives stannylenes, RaSn , which normally self react to give linear or cyclic oligomers, but if the R groups are very bulky (t-butylphenyl, or 2,6-bis(2,4,6-tri-isopropylphenyl)phenyl), kinetically stable stannylenes can be obtained (see Chapter 2.8). In the solid state, the stannylenes usually dimerize to give the distannenes, R 2Sn=SnR 2, where R denotes a bulky group. Reaction of a stannylene with a carbene gives a stannene, R 2Sn=CR2. 

Stannenes, doubly-bonded tin-carbon compounds, are viewed as bridge compounds to elucidate the similarities and differences between olefins (>C=C<) and distannenes (>Sn=Sn<). The bonding situations of stannenes are considered to be different from those of distannenes. In this section, the previously reported stannenes are classified into three types (i) typical examples, bearing three-coordinate tin and carbon atoms (except class 3), (ii) compounds bearing an Sn—C bond in a cumulative double-bond system, and (iii) compounds bearing a Sn—C bond in an aromatic ring. 

---