Crystal Structure and Mössbauer Spectroscopic Investigation of the Low Temperature Molecular Dynamics of Hexa(2-ethylimidazole)iron(II) Hydridotetracarbonylferrate(-II)

The title compound 1 was synthesized as reaction product of pentacarbonyl iron with 2-ethylimidazole. A Zerewitinoff-determination shows that we are rather dealing with hydrido-tetracar-bonylferrate(-II) than with tetracarbonylferrate(-I) as supposed in a preceding paper. A new crystal structure analysis of 1 was performed. With respect to [HFe(CO)₄]^- we found that three CO groups are ordered with normal tetrahedral angles, while the fourth CO group shows statistical distribution over three sites (disordered sites). The temperature parameters of the disordered CO sites are considerably more anisotropic than those of the ordered CO sites. Experimental Mössbauer spectra have been recorded between 4.2 K and 288.7 K. 1 exhibits a Mössbauer pattern with two subspectra, one for each of the cations and the anions. The cation subspectrum reflects normal ferrous high-spin behavior, while the anion subspectrum shows peculiar anomalies around 200 K. On the basis of our X-ray structure results molecular orbital calculations have been performed with the aim of deriving electric field gradient tensors for the various stochastic substates of [HFe(CO)₄]^-. These tensors were then used within the frame on an analytical lineshape formula. It turns out that the statistical distribution of the fourth CO group over three sites is associated with a jump behavior, similar to trapped diffusion, of the CO group among the three sites and not with static disorder. The Fe atom follows this jumb behavior, however, its triangular displacement body with jump distance d ~ 0.30 A is considerably smaller than that of the CO group.