Computed Vibrational Spectra and Structure of the Extraterrestrial Meteoritic Iron-Bearing Cyano-Carbonyl Complex [Fe^II(CN)₄(CO)₂]^2–

Here, we report the first detailed computational characterization of a hexacoordinated iron-cyano-carbonyl (ICC) complex bearing four cyano (CN^–) and two carbonyl (CO) ligands, having the general formula [Fe^II(CN)₄(CO)₂]^2– that was previously detected in Lewis Cliff 85311 Antarctic carbonaceous chondrite. The computations have been carried out using model chemistries including over ten combinations of four components: (i) density functional theory, (ii) basis set, (iii) dispersion correction, and (iv) dielectric constant, which was studied using the polarizable continuum model (PCM). Among these, the change in the polarity of the medium produced the most dramatic effect on the results of calculations. The complex may exist in two configurations: trans-ICC and cis-ICC. For the isolated complex in vacuum, the trans-ICC isomer is systematically the most stable at all theory levels, whereas the cis-ICC is higher in energy by more than 12 kJ mol^–1. Using water as a polar solvent leads to a considerable stabilization of cis-ICC, so that it becomes slightly lower in energy than trans-ICC (by 1 kJ mol^–1). The vibrational characterization of the two isomers was carried out both for the ICC in a vacuum and in the PCM. We found that the computed vibrational wavenumbers are similar in both cases (vacuum vs PCM) and for both isomers, except the CO stretching mode (νCO). The νCO wavenumbers increase by ca. 20 cm^–1 for trans-ICC, and by 30 cm^–1 for cis-ICC in the polar media (compared to vacuum). These spectral manifestations were correlated with the changes in the bond lengths of the studied complex.