FULL PAPER Polar Molecular Precursors for Alkali and Alkaline Earth Metal Clusters and Low-Dimensional Polymer Structures: the Solid-State Structures of [CaI(dme) 3 ]I, and cis-[SrI 2 (diglyme) 2 ] (dme  CH 3 OC 2 H 4 OCH 3 ; diglyme  CH 3 (OC 2 H 4 ) 2 OCH 3 ) Katharina M. Fromm* [a] and William Maudez [a] Keywords: Alkaline earth metals / Calcium / Halides / Polar compounds / Strontium Two new alkaline earth metal halide adducts with oxygen- donor polyether ligands are described. One of them, cis- [SrI 2 (diglyme) 2 ], features two terminally bonded anions in vi- cinal positions, and at an angle of ca. 90°. The low bond val- ence sum of ca. 1.9 indicates how well the cation is shielded by its ligands in the solid state, and that ligand exchange can be expected in solution and upon reaction. The second Introduction We are interested in molecular compounds, especially oxygen donor adducts of alkaline earth metal iodides, be- cause they are used as starting materials in the synthesis of low-dimensional polymeric compounds, including clusters. [1-10] The latter are used in the low-cost synthesis of superconductors and other oxide materials by the sol-gel technique, or as volatile precursors in CVD processes. [2,10] Furthermore, alkaline earth metal iodides complexed with polyether ligands have also been shown to have a dramatic effect on the dealkylation reactions of phosphinic esters. [11] We have also started to investigate the synthesis of polar materials for possible applications in NLO, for instance se- cond harmonic generation. We became interested in these compounds because we wished to understand how they were formed and discover whether polar molecules can be forced to crystallize in polar space groups. Compounds that have so far been isolated in the class of molecular species are, for instance, the zero-dimensional compounds trans-[CaI 2 (THF) 4 ] [12] or trans-[MI 2 (THF) 5 ] (M = Sr, [13] Ba [1] ), in which the metal cation is always sur- rounded in a linear or quasi-linear fashion by the two anions, whereas the THF ligands are found in the equa- torial positions of a square or pentagonal bipyramid. They are commonly used as starting materials for substitution reactions of at least one iodide. Other compounds can also be molecular, but without any direct M-I contacts left to be reactive. Instead, the halide ions are replaced in the first [a] University of Karlsruhe, Institute for Inorganic Chemistry, Engesserstr., 76128 Karlsruhe, Germany E-mail: katharina.fromm@chiam.unige.ch 3440  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ejic.200300241 Eur. J. Inorg. Chem. 2003, 3440-3444 compound, [CaI(dme) 3 ]I, consists of an ion pair formed by the cationic complex [CaI(dme) 3 ] + with one anion in terminal position, and one iodide. Both compounds possess a strong dipole moment at the molecular level, but crystallize in cen- trosymmetric space groups. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) coordination sphere by neutral water or polyether oxygen donor ligands for instance, leading to compounds such as [Ba(OH 2 )(triglyme) 2 ]I 2 [triglyme = CH 3 (OC 2 H 4 ) 3 OCH 3 ], [7] in which the anions are linked to the cationic complex by hydrogen bonding to the protons of the water molecules remaining in a second coordination sphere. We now present some new exceptional molecular com- pounds that possess either two M-I bonds in cis-positions or only one M-I bond, leading in both cases to polar mo- lecular precursors of alkaline earth metal iodides. Results and Discussion When SrI 2 is dissolved and re-crystallised from freshly dried and distilled diglyme (CH 3 OC 2 H 4 OC 2 H 4 OCH 3 ), only the compound cis-[SrI 2 (diglyme) 2 ](1) is obtained. Com- pound 1 crystallises in the monoclinic space group P2 1 /c with two independent molecules A and B per asymmetric unit (Figure 1). [14] Both molecules consist of a strontium cation to which two diglyme ligands and two anions are directly linked, with Sr-I distances between 3.314(3) and 3.335(3) A ˚ , the shorter bonds being found in the first mol- ecule A with Sr1. They are 0.11 A ˚ longer than the Sr-I bonds in other molecular strontium iodide adducts, firstly due to the higher coordination number in 1 (eight instead of seven), and secondly due to the intra-ionic repulsion be- tween the iodides in the cis-positions. [10,13] On the other hand, they are shorter than the bonds in compounds where iodide plays the role of a μ-bridging ligand between two strontium cations. [10] The I1-Sr1-I2 angle is, at 91.26(9)°, slightly smaller than that in the second molecule,