Stannylenes structures

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. 

The structure of these typical stannylenes 1 and 2 are discussed in Chapter 4 as well as that of bis(pentamethylcyclopentadienyl)tin(II) (3) which does not aggregate either but contains two r 5-bonded cyclopentadieny] ligands29), the coordination number being difficult to establish. 

As previously pointed out in Chapter 2, monomeric stannylene can be in equilibrium with oligomeric species which are formed by tin-tin or tin-substituent inter-molecular interactions. The tendency for the formation of the oligomers increases the more the molecules approach one another. Thus, when passing from the vapor to the liquid phase and finally to the solid state, the molecules usually exhibit quite different structures. In Table 13 examples of the corresponding structural changes are given. 

Apart from the determination of the structures of stannylenes by diffraction methods (X-ray or electron diffraction) many other physico-chemical techniques can be exployed to characterize these compounds more completely. Besides the classical methods such as IR-, Raman-, PE-, UV- and NMR-spectroscopy, MoBbauer-119 m-tin spectroscopy is widely used for the determination of the oxidation states of tin atoms and of their coordination 1n8-12°-123>. jt is not in the scope of this report to study the dependence of MoBbauer constants such as isomer shift and quadrupole splitting on structural parameters. Instead, we want to concentrate on one question Which information can we deduce from the structure of stannylenes to evaluate their reactivity ... 

Bulky substituents are necessary for the stabilization of stannylenes which may have different structures in solution and in the solid state. The substituents envelope the tin atom from the back side, thus restricting its reactivity. Reactions should therefore occur predominantly from the front side ... 

Stannylenes are in the first place Lewis acids (electron acceptors) as can be easily derived from the structures of the solids (Chapter 3). When no Lewis bases (electron donors) are present, they may also act as Lewis bases via their non-bonding electron pair (see polymerization of organic stannylenes). 

When stannylenes are allowed to react with nucleophiles Y the attack of the electron donor proceeds stereospecifically and orthogonal to the SnXX -plane. This can be demonstrated by the Lewis base-adducts of the stannylenes which exclusively exhibit structures of type A or B, depending on the number of nucleophiles present. 

Generally speaking, the Lewis acidity of the tin atom and, as a consequence, the stability of the formed base adduct can easily be deduced from the structure of the stannylene under consideration. If it is stable and monomolecular in non-polar solvent, it will form weak adducts. If it is not, it will form strong and stable adducts. 

The structures of these stannylene complexes closely resemble those of carbene complexes. In Fig. 12 the crystal structure of the stannylene complex 4 is displayed the tin atom, the two carbon and the chromium atoms are equi-planar 30). 

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

Insertion reactions of stannylenes, even of unstable ones, into metal-metal bonds have attracted considerable attention 156 158>. In this context, it is very astonishing that the reaction (28) between the alkyl-substituted stannylene 14 and Fe2(CO)9 does not lead to a product of type 66 (X2Sn Fe(CO)4)2 (an X-ray structural analysis indicates an Sn2Fe2-ring167)) but to a three-membered ring, as determined by elemental analysis and from IR-spectral data133). 

This outstanding behavior of bis(pentamethylcyclopendienyl)stannylene has been explained by the energetically favorable formation of the ionic compound 79 which contains the 6-membered cluster C5Sn 174>. The structure of the boron tetrafluoride compound is illustrated in Fig. 15 the tin atom in the cation is located at the apex of a pentagonal pyramide 173). 

To sum up, the geometry of the cages can well be unterstood on the basis of structural tin(II) chemistry and the nature of the bonding can be inferred from very simple approaches. The syntheses of the cages again demonstrate the various reaction possibilities of stannylenes. 

Bis(amido)-germylene and -stannylene react with [Pt(COD)2] to give the platinum(O) complexes [Pt(M NTMS 2)3], M = Ge or Sn.78 The stannylene complex has a trigonal-planar structure with Pt—Sn bond lengths of 2.47-2.50 A.79... 

This solid state structure is comparable with the first stannylene gas-phase structure reported475 by Lappert. In [(Me3Si)2CH]2Sn, the Sn-C distances were 222 pm and the CSnC angle was 97(2)°, essentially equivalent to the above. 

The compounds that are two coordinate in solution usually form SnSn-bonded dimers in the solid state, but the three- and four-coordinate stannylenes do not change their structures, and remain as monomers when they crystallize. Bis(2,4,6-tri-/-butylphenyl)stannylene is exceptional in that it is stable in the crystal, but in solution it rearranges to... 

Their structures had been much less explored than those of their heavier congeners such as germylenes and stannylenes. Although dodecamethylsilicocene (19), formally also a silylene, has been known since 1986, compound 19 is stabilized by ri -coordination of pentamethylcyclopentadienyl ligands and is not a congener of a carbene. 

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

The progress in the chemistry of plumbylenes is far behind those of its lighter congeners (silylenes, germylenes, and stannylenes). Most of the recent reports on plumbylenes are describe their synthesis and structural analysis there have been very few descriptions of their reactivities. 

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