Hexacoordinate tin complexes

In solution, the establishment of cis/trans-D equilibria in hexacoordinate tin complexes is generally fast on the H NMR time scale. A rare observation of different isomers of neutral octahedral diorganotin compounds in solution was described1002. This is somewhat surprising, but may be traced to the usually poor solubility of hexacoordinate organotin compounds, making low-temperature NMR studies difficult if not impossible. 

From Me2SiCl2 and 2 cis-hexacoordinated 9 is isolated (Eq.(5)) whose structure may be compared to the analogous tin complex 10 (see Table 1). 

A special feature of anions 475 in complexes with different counterions (entries 4-16) is a lengthening of the N — Sn or O — Sn coordinative bonds up to 2.28-2.32 and 2.17-2.27 A, respectively, compared to the mean values for penta- and hexacoordinated tin compounds (2.12 and 2.15 A)9. Accordingly, the Sn—Cl bond length trans to N or O, at 2.36-2.42 A, is somewhat shorter than that in hypervalent compounds (2.45 A), and is generally shorter in comparison with the other Sn—Cl bonds in these complexes. 

TABLE 67. X-ray data for anions R2>SnX and RSnXj in hexacoordinate anionic tin complexes and related species"... 

Almost aU these complexes had a hexacoordinated tin atom in octahedral environment. It is remarkable that only five intermolecular coordination compounds Me3SnX B (Cl, Br, I B = Py, PhNH2 ) containing pentacoordinated tin atom... 

The molecular structure analyses of 3a and 3b (Fig. 3) confirm the hexacoordination of the Si atom. The coordination sphere around the Si atom is distorted octahedral. In both cases the monodentate ligands (phenyl group as well as benzoate and picrate) are situated in the trans position. This is probably typical for Salen-Si complexes [3, 4], but their configuration is quite different from that of the hexacoordinate tin compound 2c. 

The knowledge of d( Sn) chemical shifts is very important for predicting the geometry of coordination polyhedra of tin compounds in solution. In the case of tetravalent tin compounds, an increase in tin coordination number from four to five corresponds to an increase in tin shielding by -150-200 ppm. Similar increases are generally associated with six and seven coordination. The (5( Sn) for PhSn(edtc)2Br [2], a hexacoordinate pseudooctahedral tin complex, is -704.1 ppm, whereas d( Sn) for PhSn(edtc)3 [3] in which the central Sn has coordination number seven and the shape is distorted pentagonal bipyra-midal is -806.5 ppm. 

Quite recently, it was established that the formation of the pentacoordinated germanium and tin compounds EF4(CH2CH=CH2) from EF3(CH2CH=CH2) (E = Ge, Sn) by the addition of F is exothermic148. The nucleophilicity of the allylic y-carbon is much enhanced when the pentacoordinated EF4(CH2CH=CH2) is formed. These results are qualitatively similar to those found for the reaction of the corresponding silicon compound. The pentacoordinated Ge and Sn complexes have a significant Lewis acidity which allows them to form stable hexacoordinated intermediates in the course of the reaction. 

Diaryltin dicupferronates have been isolated by the reactions of acetonic solutions of diaryltin dichloride with a solution of cupferron in the same solvent.123 On the basis of IR spectra, a hexacoordinate geometry is suggested for tin in these complexes. 

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