ALKOXIGERMANATES(II), STANNATES(II) AND PLUMBATES(II) OF 2-VALENT METAL-IONS .1.

Bis(tri-tert-butoxigermanates), -stannates and -plumbates of Mg (1a), Ca (1b, 2b, 3b), Sr (1c, 2c, 3c), Ba (1d, 2d, 3d), Cd (1e, 2e, 3e), Eu (1f, 2f, 3f) and Pb (1g, 2g, 3g) have been synthesized by various procedures from starting materials such as sodium tri-tert-butoxigermanate (5) and -plumbate (6) and other analogous compounds. While in 1g and 2g the lead atom is situated in the middle of the molecule, it can also occupy two different coordination sites as in Pb(OtBu)3Pb(OtBu)3Pb (3g) and in Ge(OtBu)3Pb(OtBu)3Pb (7). X-ray structure investigations on 1a, 1b, 1e, 1f, 1g, 2b, 2e, 2g,3c, 3d and 3g reveal that all these molecules have a common ElO3MO3El cage unit. This cage unit can be visualized as two ElO3M trigonal bi-pyramids sharing the central metal M. Consequently in all structures the central element M is sixfold coordinated by oxygen while the outer metals (El) have three oxygen neighbours. The oxygen atoms are further bonded to tert-butyl-groups, which surround the whole ElO3MO3El frame. Four different structure types have been found to exist: a monoclinic type-alpha (1a), an orthorhombic type-beta (1b, 1e, 2b, 2e), a rhombohedral type-gamma (1f, 1g, 3c, 3d, 3g) and a monoclinic type-delta (2g). In the beta-structure type the molecules are found to exist as three different isomers, depending on the arrangement of the alkoxy groups in a right-or left-handed manner within the El(OtBu)3 groups (RR, SS and RS). Solution equilibria between these isomers are observed by NMR. In the case of 1e a more detailed picture of this intramolecular motion, attributed to the inversion of the trigonal-pyramidally coordinated oxygen atoms, can be given. CP-MAS-NMR -spectra as well as solution spectra have been obtained for 1e (C-13, Cd-113), 2e (C-13, Cd-113, Sn-119), 2g (Sn-119, Pb-207), 3g (C-13). While delta-Cd-113 values of 2e in solution and in the solid are comparable (92.8 vs 73.7 ppm), the 2J(Sn-119/Cd-113) values rise from 39 Hz to 295 Hz. Detailed comparison of bond lengths and angles in the different derivatives shows that the "softness" of the El(OtBu)3 groups (or the ability of the groups to accommodate a metal atom with respect to its size) follows the order Sn(OtBu)3 < Ge(OtBu)3 < Pb(OtBu)3. From these findings it can be explained, why the lead derivative 2g shows stereochemical activity of the lone pair, while 1g and 3g do not show this effect at room temperature. All compounds except the lead derivatives 1g, 2g, 3g and 7 are colourless.