Sandwich complexes crystalline structures

From solutions of GaCl2 in hot 1,3,5-trimethylbenzene the colorless crystalline bis(arene) complex [C6H3(CH3)3]2Ga+ GaCl4 (XLVI) has been isolated (158). The structural study reveals a bent sandwich structure for the cationic moiety, as indicated schematically in Fig. 9. The tetrachlorogallate tetrahedra are once more only weakly coordinated, this time via one chlorine atom each to the Ga(I) centers of two bis(arene)gallium units, forming a chainlike coordination polymer. The steric requirement of the mesityl ligands is... 

X-ray analysis of the crystalline 2 2 complex of beauvericin with Ba++ has revealed the interesting structure shown in Fig. 32118 The two Ba++ ions in the center of the complex are held together by three picrate anions which contribute to the coordination. Each of the two beauvericin molecules contributes three chelating amide oxygens. The resulting structure has much in common with the sandwich conformations shown in Fig. 30. 

After beryllocene was first prepared in 1959, early work showed its IR spectmm to be more complex than those of the symmetrical sandwich molecules such as ferrocene, and it was also found to have a large dipole moment in solution, 2.24 D in cyclohexane. It was thus clear from the start that the molecular symmetry was lower than that of ferrocene, though it was not clear whether this involved two dissimilar rings or two identical but asymmetric rings. An early X-ray diffraction study appeared to show a unit-cell structure formally identical to that of ferrocene, implying a centrosymmetric molecule (12.IVa), which is incompatible with the observation of a dipole moment in solution. Thus there was a possibility that the molecular structure in the crystalline phase was not the same as that in gaseous and solution phases, but it was also possible that the crystallographic results could be attributed to disorder in the crystal. 

Terphenyl iron and cobalt bromide derivatives were studied with a view to improving yields and crystallinity. The iron species Li(Et20)Ar FeBr2 2 was obtained as an amber powder from hexanes or benzene, but it did not crystallize well. However, 3, 5- Pr2-Ar Fe(p,-Br) 2 (24) was prepared by the reaction of 3, 5- Pr2-Ar Li with FeBr2 in diethyl ether and structurally characterized. This complex could also be obtained by oxidizing the half sandwich Fe(I) complex 3, 5- Pr2-Ar Fe(ri -C6H6) with BrN(SiMc3)2 [41] (Fig.4). For further details see Sect. 8.2. 

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