Structure stannenes

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

While the chemistry of silenes R2Si=CR2 has been well developed, structural information about germenes R2Ge=CR2 and stannenes R2Sn=CR2 is still scarce, and no plumbene R2Pb=CR2 had been isolated to date. 

Scheme 28)66. An X-ray structural analysis of 75 revealed the presence of a long tin-carbon double bond with a length of 2.379 A. The geometry of the molecule observed here clearly indicates that the adduct 75 is better described as the limiting formula 75b rather than as the stannene form 75a. 

Because their structural characteristic is an unsaturated silicon, germanium, or tin atom >E=, they may, in the widest sense, be named silenes, germenes, or stannenes. As a rule, they are unstable imder normal conditions and isolable only as dimers, but they are formed as intermediates from suibd>le initial stages by cycloreversion or salt elimination (Scheme 2) [1,3-11]. 

Two remarkable reactions are shown in equations 21-13 and 21-14. The stannylene 21-1 reacts with the cycloalkyne 21-28 to give the adduct 21-29 [5(119Sn) -536.8] which in solution is in equilibrium with its factors, but can be isolated as a pale yellow solid. It has rSnC 213.6(5) and 213.4(5) pm, and rC=C 134.0 pm, and its structure is probably best described as a 7i-complex on the Dewar-Chatt-Duncanson model.60 The same stannylene reacts with the boranediylborirane 21-30, which behaves as a pseudo-carbene, to give the stannene 21-31 (see Section 21-2).46... 

The Sn=X double bond can be formed either by the coupling of a stannylene R2Sn with an equivalent species AB with 6 valence electrons (equation 21-17), or by the elimination of MX from the structure MSn-AX (M = alkali metal, X = halide) (equation 21-18). Examples of the stannenes and heterostannenes are shown in Scheme 21-10. 

The structures of stannenes 22-30 were determined by X-ray crystallographic or NMR spectroscopic analyses, and some of their parameters are summarized in Table 2.9.2. The molecular structures of 22 and 27 are shown in Figures 2.9.4a and b, respectively. 

Advances in techniques include sophisticated NMR methods, both in solution and the solid-state, and computational methods, and these, coupled with X-ray diffraction and other established methods, have been applied to the study of a wide variety of structures. Topics covered in the book include Sn(II) clusters, tin Zintl ions, Sn(II) heterobimetallic compounds, RsSn+ cations, stannylenes (R2Sn ), stannenes (R2Sn=SnR2 and R2Sn=CR2), stannynes (RSn=SnR), organotin oxide, carboxylate and sulfonate clusters, dendrimers and macrocycles, organotin polymers, Sh-tt interactions, unusual bondings and structures, and compounds with non-linear optical properties. 

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