Silole dianions structure

The known silole dianions are coordinated to the metallic counter ions either by rj5,rj5-bonding10a 82 or by r/1, r/ -bondinglob (Figures 8c and 8d). Pyramidality at Si is, of course, not a problem for the silole dianions where the Si is dicoordinated, and the known dianions are all planar (Figures 8c and 8d). The structural and magnetic properties of the silole dianions are consistent with charge delocalization and aromatic character of these species, as shown by the data collected in Table 7. 

The dicoordinated silicon in the aromatic silole dianion 574 resonates at an nnexpectedly low field for a sUyl dianion (5 Si = 68.5 ppm) . The diffeent chanical shifts in the solid state (5 Si = 87.3 ppm) and in solntion suggest diffaent structures. While in the solid state a structuie 574a is proven by an X-ray structure, in solution a highly symmetric structure 574b seems to dominate . According to ab initio calculations both isomers show indications of cyclic conjugation . ... 

TABLE 7. Experimental structural and magnetic properties of silole anions and dianions and conclusions regarding their degree of aromaticity... 

The solid-state structure of the silolyl dianion is substantially affected by the nature of the substituents attached to the silole ring and of the metal alkali counterions. Stirring C4Me4SiBr2 with four equivalents of potassium in the presence of 18-crown-6 gives... 

In summary, the anions of group 14 metalloles (C4E rings) have either localized nonaromatic or delocalized aromatic structures, depending of the metal E, Si vs Ge and Sn, and on the substituents, methyl vs phenyl group. As pointed out by West and coworkers198, structural studies of metallole anions and dianions of this kind with different substituents will be of value. Additionally, it should be noted that experimentally observed structures in the solid state for silole and germole dianions are greatly influenced by the nature of the alkali metal counterion. 

Alkali metal reduction of dihalide precursors is shown to be a valuable route to various silole and germole dianions in solution. Crystal structure determinations for [K(18-crown-6)+h(C4Me4Si2 ] and the dimer [K4(18-crown-6)3][C4Me4Ge]2 are consistent with the presence of delocalized jr-systems and with -bonding modes for all of the potassium cations. 

Summary According to structural, thermodynamic, electronic, and magnetic criteria silole and germole dianions show indications of cyclic delocalization. The degree of aromaticity in the silole- l(Si) and germole dianions l(Ge) is of the same magnitude as that in the cyclopentadienyl anion. 

The silole- l(Si) and the germole dianions l(Ge) are calculated to be planar (inner-cyclic dihedral angle S = 0°, see Table 1), The structural data, summarized in Table 1, clearly indicate the occurence of cyclic delocalization in the dianions. Thus, the C-C bond distances in the butadiene part of the... 

These considerations led to the preparation of the ti -coordinated complexes of siloles and germoles (94PAC1431). A number of facts in support of this view were obtained by X-ray determination of the structures of the organometallic derivatives of nontransition metals. Thus, the dianion of tetraphenylgermole forms a sandwich (Ulithium salt264 in dioxane at —20°C [96AG(E)1002]. At 25°C, however, the salt with a mixed coordination mode 265 results. Columnar structure is observed... 

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