The crystal structure of [Et4N][CoFe3 (CO) 13] has been determined, and shown to contain an anion with the CO ligands adopting a very different arrangement about the tetrahedral CoFe3 core to that found for the same anion, reported previously for the PPN+ salt. The relationship between the two structures can be understood in terms of Johnson's Ligand Polyhedral Model. In contrast the anion in [Et4N][CoRu3 (CO) 13] has the same geometry as that described in the corresponding PPN+ salt.
