Revisiting the Vibrational Force Field of Matrix-Isolated [Fe(CO)<sub>4</sub>]<sup>-</sup>

<jats:p>The 17-e anion, [Fe(CO)4]- , was first characterised in 1981 by Breeze et al. We subsequently challenged their interpretation, suggesting that they had produced [Fe(CO)3]-. In the light of new calculations here, we withdraw that view. We confirm that [Fe(CO)4]- has the C3v structure (as previously reported) and we have calculated the following, new, energy-factored force constants: k1 = 1437.2 k2 = 1456.2 k11 = 48.6 k12 = 47.7 where k1 refers to the three chemically identical equatorial CO groups and k2 refers to the sole axial CO group ( k11 and k12 are the CO,CO interaction constants). This force field is significantly different from that originally proposed.</jats:p>