Divalent tin compounds

Scheme 2 Synthesis of five-coordinated tin tetrasulfido complexes from N-coordinated divalent tin compounds...

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

The characteristic feature of the divalent tin compounds is that they have both a vacant orbital and a lone pair of electrons (Figure 2.5). In this glycosylation reaction, it is assumed that SnCl2 behaves as a Lewis acid, where the vacant orbital accepts one of the three lone pairs in the fluorine atom of the glycosyl fluoride. As a result of this interaction, the C—F bond cleaves to give the oxocarbenium ion intermediate that is then attacked by an alcohol to give the glycoside. 

The problem of the sign of AR/R for the divalent tin compounds was investigated by Lees and Flinn (16). In the relationship between the quadrupole splitting and chemical shift for the stannous compounds, two distinct correlations became apparent—compounds with a linear covalent bond, and compounds with a predominantly planar bond. Furthermore, there exists a linear relationship between the number of 5 p electrons and the chemical shift and hence the total 5 electron density. Using free tin ion wave functions in a self-consistent field calculation, they showed that the direct eflEect of adding 5 electrons is considerably... 

Protolytic cleavage of the Sn—Cp bond of stannocene is a reaction of great synthetic utility in the synthesis of a variety of divalent tin compounds. In some cases the partially substituted product CpSnX can be isolated, but in most reports SnX2 is synthesized according to Eq. (24) (43-49). 

Early attempts at producing dialkyltin compounds yielded polymers. More recently, Neumann has found several synthetic routes to reactive R2Sn intermediates which can be trapped by oxidative-addition reactions (J). In the absence of trapping agents the divalent tin compound polymerizes. Lappert and co-workers have shown that the bulky bistri-methylsilylmethyl ligand stabilizes the divalent tin species toward polymerization. This stable divalent tin species thus provides an excellent starting material for investigating a wide variety of oxidative-addition reactions, as shown in Fig. 10 (78). 

The Mossbauer data for a number of sigma bonded divalent tin compounds are collected in Table V (64, 72, 74, 77, 88, 89). The reader is referred to reference 74 for a more complete list of divalent amides we have listed here only Sn[N(SiMe3 )2]2 for comparison to the isoelectronic alkyl compound. The exceptionally low value for the divalent compound Sn[CH(SiMe3)2]2 is no doubt due to its unusual dimeric structure in the solid state. All the other compounds listed in Table V as divalent structures have isomer shifts above the often-used dividing line of 2.56 mm/sec for tin(II). 

In addition to serving as ligands, organostannylenes can also insert into metal-ligand bonds (cf. Fig. 10). In the reaction shown in Eq. (50) (78), two moles of the divalent tin compound react, one forming a terminal stannylene complex, and the other inserting into the Pt—Cl bond. 

Figure 2 The angle (0 ) formed between the substituents in divalent tin compounds...

Tin(II) can act as a donor. Complexation of various divalent tin compounds by Lewis acids has been realized, many in transition metal complexes, e g. (15) and (16), but also with main group Lewis acids, e g. (16). 

Fig. 18 Divalent tin compounds bearing bulky dithiocarboxylate ligands...

The synthesis of doubly bonded tin compounds by the coupling of stannylenes, however useful, is limited by the need for a stable stannylene and often a second divalent species (for example, a carbene or isonitrile). The simplest example of this reaction is the formation of tetrakis[bis(tri-methylsilyl)methyl]distannene from two molecules of the corresponding stannylene,83 with which it is in equilibrium in solution as evidenced by NMR spectroscopy.91... 

The cyclic diazastannylene 1 has been found to be very suitable for this type of reaction 1S5) (cf. also Sect. 4.1). In Eqs. (43) and (44) the chlorine atoms of the Lewis acids are transferred to the divalent tin atom resulting in the formation of 57 and 76 and tin(II) chloride, the latter being insoluble in benzene. In (45) the solubility of the produced compounds is again important because SnS precipitates from the solution thus, the equilibrium is shifted to the right (in Eqs. (43)-(45) R denotes tert-butyl). 

The dihalides of tin and lead are so very well known that it is unnecessary to summarize the extensive chemical knowledge of these compounds. The chemistry of divalent tin and lead has been reviewed several times recently 132 A few points that are relevant to the material already discussed will be made. 

Brother Columba (University of Notre Dame) With regard to earlier discussion on whether the data all support that divalent tin is positive, we have run about 30 coordination compounds of divalent tin, and all the isomer shifts are strongly positive. 

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... 

V. I. Shiryaev, V. F. Mironov and V. P. Kochergin, Compounds of Divalent Tin and Synthesis of Organotin Compounds, NIITEKhIM, Moscow, 1977 (Russian). 

While the inorganic chemistry of divalent tin and lead is well established, there are few well-characterized divalent organotin and organo-lead compounds. The R2Sn formulation seen in the older literature has proved to be an empirical formula, the formally divalent stannylene actually being an oligomeric linear or cyclic organostannane. Neumann has commented on this point several times (1-3). 

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