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The crystal and molecular structure of a stable -arene-chromium dicarbonyl complex of bis(diphenylarsino)methane (DAM) [{0r-C6Hs)(C6H5)AsCH2As(C6Hs)3 ]Cr(CO)3] has been determined by three-dimensional X-ray structural analysis, using data collected by counter methods. The complex crystallizes in the space group PI (C¡', No. 2), a = 7.609 (3), b = 8.563 (3), c = 18.677 (5) A, a = 83.16 (2), ß = 82,52 (2), = 81.66 (2)°, Z = 2. The measured and calculated densities of 1.62 (1) and 1.622 g cm"3, respectively, are in excellent agreement. The structure was solved by conventional Patterson and Fourier techniques and refined by block-diagonal least-squares methods to final weighted and unweighted reliability indices of 0.042 and 0.032, respectively, for the 3208 independent reflections for which l/o(f) > 3.0. One As atom is disordered, and the populations of the two possible positions have been successfully refined to 0.9510 (9) and 0,0582 (9). The coordination at the central chromium atom is essentially octahedral, with the two carbonyl groups and one -bonded As atom of the DAM ligand occupying three mutually cis coordination sites. The remaining sites at the chromium atom are occupied by a delocalized -arene ring which is also bonded to the second As atom of the DAM ligand. Although the Cr-C-(arene) distances vary from 2.165 (4) to 2.194 (4) A, the Cr-^-arene bonding appears to be essentially symmetric. The shortest distances are approximately trans to the -bonded arsenic atom. The -arene ring is planar within experimental error, and there is no systematic alternation of C-C bond distances within this ring.

Raman spectra of I<,SnC16 are investigated in the temperature range between 50 and 300 K. The observed low temperature spectra are compatible with the proposed monorlinic structure. Two bands that exhibit a soft behaviour are assigned to a zone contor and zone boundary rotational mode, respectively. It is supposed that these modes drive the structural phase transitions a t T , = 263 K and T , = 255 K.Die Raman-Spektren von K,SnC16 werden im Temperaturbereich von 50 bis 300 K untersucht.Das beobachtete Tieftemperatur-Spektrum steht in Einklang mit der vermuteten monoklinen Strulitur. Zwei Bandon, die ein Soft-Verhalten xeigen, werden Rotationsmoden zugeordnet, die zum Zentrum bzw. zum Rand der Brillouin-Zone gehoren. Diese Moden werden als Ursache fur die strukturellen Phasenubergange bei T, = 263 K und T z = 255 K angesehen.

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