Reaction between thiamine diphosphate hydrochloride (H 3 TDPCl) and methylphenyltin dichloride in water at pH ca. 5.6 afforded the title complex, which was studied by X-ray diffraction and IR and Raman spectroscopy in the solid state and by multinuclear NMR spectroscopy ( 1 H, 13 C, 31 P, and 119 Sn) in aqueous solution. The crystals of this compound are formed by the oxygen-capped cluster cation [{SnMe(HTDP)(OH)} 3 O] + , an OHcounterion, and water of crystallization. The cation has 3-fold symmetry, and the tin atom has coordination number 6 and a distorted octahedral coordination polyhedron. The HTDPanion is bis-monodentate bridging between two Sn atoms via two of the oxygens belonging to the terminal phosphate of the ethyl diphosphate side chain [Sn-O ) 2.074(3), 2.100( 5) Å]. The thiamine moiety has F conformation, with torsion angles C(2)-N(3)-C(3,5′)-C(5′) ) -5.8(11)°and N(3)-C(3,5′)-C(5')-C( 4′) ) -83.0(10)°, and the ethyl diphosphate side chain is folded back, directing the pyrimidine and thiazolium rings of the three HTDPanions away from the "missing" corner of the Sn 3 O 4 core so that they form the walls of a "nest" with the core as its floor. Although the diffraction data do not allow direct location of most of the hydrogen atoms, indirect structural evidence suggests (i) that the OHcounterion lies inside the nest on the 3-fold symmetry axis, (ii) that the remaining dissociable proton of the HTDPanion forms a strong hydrogen bond between the pyrimidine N(1′) atom and the uncoordinated oxygen of the terminal phosphate of another HTDPbelonging to the same cluster, and (iii) that one of the oxygens of the nonterminal phosphate is hydrogen-bonded to one of the -OH groups that bridge between the Sn atoms. Features ii and iii are probably responsible for the folding of the ethyl diphosphate side chain. Hydrogen bonds involving the N(4′R)-H 2 group on the pyrimidine ring and the water molecules of crystallization are also present. The multinuclear NMR results indicate that most of the structural features exhibited by the cluster in the solid state remain in D 2 O solution. IR and Raman spectra are also discussed.