The syntheses of the asymmetrically substituted tetraorganodistannoxanes [t-Bu 2 (X)SnOSn(Y)(CH 2 SiMe 3 ) 2 ] 2 (1, X = Y = OH; 2, X = Cl, Y = OH; 3, X = Y = Cl) are reported and their structures in solution and in the solid state are characterized by multinuclear NMR spectroscopy and single crystal X-ray analyses. In toluene, the tetrahydroxysubstituted derivative 1 is in equilibrium with the organotin oxides cyclo-[t-Bu 2 Sn{OSn(CH 2 SiMe 3 ) 2 } 2 O] (4), cyclo-[(Me 3 SiCH 2 ) 2 Sn(OSnt-Bu 2 ) 2 O] (5), and cyclo-(t-Bu 2 SnO) 3 , and some additional, undefined species containing pentacoordinated tin atoms. In contrast, the dihydroxydichlorosubstituted derivative 2 is inert in solution.

The synthesis of the stannasiloxanes and [Cp(CO) 2 Fe] 2 Sn(OSiPh 3 ) 2 (7a), the monoorganotin trichloride Me 3 SiCH 2 SnCl 3 (13), and the organotin oxocluster (Me 3 SiCH 2 -Sn) 12 O 14 (OH) 6 Cl 2 ( 14) is reported. Their identity was confirmed by both solution and solid state multinuclear NMR spectroscopy and in the case of 1, 2, 6, 7a, and 14 also by singlecrystal X-ray analyses. A spinning sideband analysis of the 119 Sn MAS spectra reveals the coordination geometries of the tin atoms in the stannasiloxanes 1 and 2 to be different from those of related diorganotin oxides (R 2 SnO) n (R ) Me, Et). In solution, 1 and 2 exhibit an intramolecular exchange process as well as monomer-dimer equilibria. The reaction of 4 with cyclo-(t-Bu 2 SnO) 3 and of cyclo-[(Me 3 SiCH 2 ) 2 SnO] 3 with cyclo-(t-Bu 2 SnO) 3 provided evidence for the formation in situ of the mixed tetraorganodistannoxane t-Bu 2 (Me 3 SiO)-SnOSn(OSiMe 3 )(CH 2 SiMe 3 ) 2 (10) and of the hexaorganotristannoxanes cyclo-[(Me 3 SiCH 2 ) 2 -Sn(OSnt-Bu 2 ) 2 O] (11) and cyclo-{t-Bu 2 Sn[OSn(CH 2 SiMe 3 ) 2 ] 2 O} (12).

Small amounts of [Ph 2 (OH)Sn] 2 CH 2 effectively catalyze the condensation of Ph 2 Si(OH) 2 to give cyclo-(Ph 2 SiO) 4 in good yield. The reaction proceeds under remarkably mild conditions. In contrast, the reaction of [Ph 2 (OH)Sn] 2 CH 2 with Ph 2 Si(OH) 2 in a stoichiometric ratio quantitatively gives the six-membered stannasiloxane ring cyclo-Ph 2 Si(OSnPh 2 ) 2 CH 2 (1). Compound 1 reacts slowly and irreversibly with Ph 2 Si(OH) 2 to provide the eight-membered stannasiloxane ring cyclo-O(Ph 2 SiOSnPh 2 ) 2 CH 2 (2) and [Ph 2 (OH)Sn] 2 CH 2 . The driving force for this reaction is attributed to the release of ring strain in 1. Condensation of [Ph 2 (OH)Sn] 2 -CH 2 with diphenylsiloxanols HO(Ph 2 SiO) n H (n ) 2-4), t-Bu 2 Si(OH) 2 , and t-Bu 2 Ge(OH) 2 , respectively, provides the metallastannoxane rings cyclo-O(Ph 2 SiOSnPh 2 ) 2 CH 2 (2), cycloPh 2 Si(OSiPh 2 OSnPh 2 ) 2 CH 2 (3), cyclo-O(Ph 2 SiOPh 2 SiOSnPh 2 ) 2 CH 2 (4), cyclo-t-Bu 2 Si(OSnPh 2 ) 2 -CH 2 (5), and cyclo-t-Bu 2 Ge(OSnPh 2 ) 2 CH 2 (6). The molecular structures of 2 and 5 were determined by X-ray diffraction.

The single-crystal X-ray structure analysis of hexakis(2,4,6-triisopropylphenyl)cyclotristannoxane, cyclo-[(2,4,6-i-Pr 3 -C 6 H 2 ) 2 SnO] 3 (1), is reported and reveals this compound to contain an almost planar six-membered ring. Redistribution reactions of 1 with cyclo-(t-Bu 2 SnO) 3 and tBu 2 SiCl 2 , respectively, failed and indicate an unusual kinetic inertness of the Sn±O bonds in 1 as compared to related molecular diorganotin oxides containing less bulkier substituents. The redistribution reaction of cyclo-(t-Bu 2 SnO) 3 with cyclo-(t-Bu 2 SnS) 2 leads to an equilibrium involving the trimeric diorganotin oxysulphides cyclo-t-Bu 2 Sn(OSnt-Bu 2 ) 2 S (2 a) and cyclo-t-Bu 2 Sn(SSnt-Bu 2 ) 2 O (2 b).

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