Metal-metal bonds between group 12 metals and tin: Structural characterization of the complete series of Sn-M-Sn (M = Zn, Cd, Hg) heterodimetallic complexes

Reaction of the lithium triamidostannate [MeSi{SiMe2N(p-Tol)}(3)-SnLi(OEt2)] (1) with 0.5 molar equivalents of MCl2 (M = Zn, Cd, Hg) in toluene afforded the corresponding heterodimetallic complexes [MeSi{SiMeN(p-Tol)}(3)Sn](2)M {M = Hg (2), Cd (3), and Zn (4)}. The molecular structures of the mercury and cadmium complexes were determined by X-ray diffraction and found to adopt a linear Sn-M-Sn metal-metal bonded array (d(Sn-Hg) 2.6495(2), d(Sn-Cd) 2.6758(1) Angstrom), these being the first Hg-Sn and Cd-Sn bonds to be characterized by X-ray diffraction. That the Hg-Sn bonds are shorter than the Cd-Sn bonds in the isomorphous complexes is attributed to relativistic effects in the mercury system. In contrast, the structure of the Zn analogue is unsymmetrical with one stannate unit being Sn-Zn bonded (d(Sn(1)-Zn) 2.5782(4) Angstrom), while the Zn-II atom bridges two amido functions of the second stannate cage, thus representing a second isomeric form of these complexes. The different degree of metal-metal bond polarity is reflected in the Sn-119 NMR chemical shifts of the three complexes. Variable-temperature NMR studies and a series of H-1 ROESY experiments of the cadmium complex 3 in solution revealed a dynamic exchange between the two isomers.