= Solution
The wording places both dicarbollide cages on the same <iron>, leaving the second <iron> outside the cages. The corresponding proposal is \b[$[L_2\mathrm{Fe\rightarrow Fe(CO)_4}]^{2-}$], with the first <iron> sandwiched between the two open $\eta^5$ faces and the second carrying four carbonyls. It is not the symmetric arrangement with one cage on each <iron>, which would contradict the specified single cage-interacting metal site.
The donor fragment $[\mathrm{Fe^{II}}L_2]^{2-}$ has $d^6+2(6)=18$ electrons; the external neutral $\mathrm{Fe(CO)_4}$ fragment has $8+4(2)=16$. Metal-to-metal donation therefore provides a natural 18-electron acceptor adduct. In covalent-single-bond oxidation-state bookkeeping, the analogous assignments Fe(III)/Fe(−I) give $d^5+12+1=18$ and $d^9+8+1=18$. In either account the whole species has two negative charges. The outside <iron> can have an approximately trigonal-bipyramidal five-coordinate arrangement when the Fe–Fe direction and its four CO ligands are counted.
The important structural distinction is \b[a bis(dicarbollide) <iron> donor joined to an exo tetracarbonyl <iron> acceptor]. Formula and <electron counting> alone do not uniquely fix all <carbonyl> angles or prove a particular crystalline conformation.
Back to article page
