Intramolecular coordination oxygen atom

Also in compounds 86-89, intramolecular Sn—O coordination is relatively weak as was shown by replacement of the intramolecular coordinating oxygen atom by external ligands like pyridine 139,140). 

That the intramolecular O Sn coordination in compounds 320a-c is relatively weak was also shown by replacing the intramolecularly coordinating oxygen by the external strong Lewis base BU3PO to yield 1 1 or 1 2 adducts. In the latter, the tin atom remains pentacoordinated and the added Lewis base displaces a halide group. The tendency to form the 1 2 adduct decreases in the sequence I > Br > Cl. The Cl derivative forms no adduct. 

An X-ray crystal structure determination of (2-carbomethoxy-l,4-cyclo-hexadien-l-yl)trimethyltin, 30, revealed that the tin center in this compound is pentacoordinate as a result of intramolecular Sn—O coordination see Fig. 9 (99). The C(l) atom of the cyclohexadiene moiety and two of the methyl groups are at approximately equatorial positions, and the other methyl group and the coordinating oxygen atom [Sn—O distance 2.781(3) A] are at the axial sites of a distorted trigonal bipyramid. 

This oxidation of DMSO is catalyzed by Ag+ cations. Kinetic and infrared spec-trometric evidence fits a mechanism where DMSO coordinates rapidly with Ag+ through its oxygen atom. The oxidation of this complex by Ce4 + then constitutes the slow step. The Ag2+ adduct would then undergo an intramolecular electron transfer in a fast step resulting in the oxidation of DMSO. 

Few X-ray diffraction data are available for the organotin alkoxides. Trimethyltin methoxide in the crystal is a five-coordinate linear polymer 47,380 and both tin atoms in the methanol solvate, CH2(SnPh2OMe)2-MeOH, are five coordinate, one by intramolecular coordination, and the other by solvation 48.381 Dimethyltin dimethoxide 49,382 dimethyltin diphenoxide 50,383 and isopropyltin triisopropoxide 51384 are five-coordinate oxygen-bridged dimers. All these compounds are monomers in solution. 

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