The molecular structure of (r¡5-CsH5)2Fe2(CO)3(GeMe2) has been determined crystallographically and the structural and dynamical properties of the molecule in solution have been studied by infrared and nuclear ('H, 13C) spectroscopy. The compound crystallizes in the space groupP212i2i with a = 13.503 (1) A, b = 14.162 (2) A, and c = 8.229 (1) A, and with Z = 4. From anisotropic refinement of 2141 reflections with intensities greater than their standard deviations the structure was solved and refined to/?, =0.035 and/?2 =0.048. A number of Friedel pairs were carefully measured and the correct enantiomorph was unequivocally selected. The molecule has a bridging Me2Ge group and is the cis isomer of (ps-CsH5)2Fe2-(CO)2(µ-CO)(µ-Me2Ge). Each bridging group is symmetrical and the Fe-Fe distance is 2.628 (1) A. In solution at 25°t here is an approximately 8:1 cis-trans mixture, with the isomers interconverting too slowly to influence either 'H or 13C Une shapes. Between 90 and 160°(above which there is decomposition giving rise to paramagnetic impurities) the pmr signals for both CH3 and n5-CsHs groups collapse and coalesce. The results are best explained by a process in which concerted opening of the two bridges occurs, leading to a nonbridged intermediate which can undergo internal rotation and then reestablish bridges. This mechanism accounts in detail for all the experimental observations. The high activation energy, ca. 21 kcal mol"1, is attributed to the relative instability of the intermediate which contains a terminal dimethylgermylene Ugand.

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