Abstract
<jats:p> The 17-e anion, [Fe(CO) <jats:sub>4</jats:sub> ] <jats:sup>-</jats:sup> , was first characterised in 1981 by Breeze et al <jats:sup>1</jats:sup> . We subsequently challenged their interpretation <jats:sup>2</jats:sup> , suggesting that they had produced [Fe(CO) <jats:sub>3</jats:sub> ] <jats:sup>-</jats:sup> . In the light of new calculations here, we withdraw that view. We confirm that has the structure (as previously reported) and we have calculated the following, new, energy-factored force constants: </jats:p> <jats:p> <jats:italic toggle="yes">k</jats:italic> <jats:sub>1</jats:sub> = 1437.2 <jats:italic toggle="yes">k</jats:italic> <jats:sub>2</jats:sub> = 1456.2 <jats:italic toggle="yes">k</jats:italic> <jats:sub>11</jats:sub> = 48.6 <jats:italic toggle="yes">k</jats:italic> <jats:sub>12</jats:sub> = 47.7 </jats:p> <jats:p> where <jats:italic toggle="yes">k</jats:italic> <jats:sub>1</jats:sub> refers to the three chemically identical equatorial CO groups and <jats:italic toggle="yes">k</jats:italic> <jats:sub>2</jats:sub> refers to the sole axial CO group ( <jats:italic toggle="yes">k</jats:italic> <jats:sub>11</jats:sub> and <jats:italic toggle="yes">k</jats:italic> <jats:sub>12</jats:sub> are the CO,CO interaction constants). This force field is significantly different from that originally proposed. </jats:p>