Binuclear homoleptic iron carbonyls: Incorporation of formal iron-iron single, double, triple, and quadruple bonds, Fe-2(CO)(x) (x = 9, 8, 7, 6)

The homoleptic binuclear compound Fe-2(CO)(9) is well characterized experim entally, although there has been some discussion as to the nature of the ir on-iron bond, which is at most a single bond. In this research, we consider homoleptic iron carbonyls that satisfy the 18-electron rule but may have f ormal double Fe-2(CO)(8) (seven distinct structures), triple Fe-2(CO)(7) (t hree distinct structures), and even quadruple Fe-2(CO)(6) (seven distinct s tructures) iron-iron bonds. These novel structures are characterized in ter ms of their equilibrium geometries, thermochemistry, and vibrational freque ncies. The range of predicted iron-iron distances is remarkable, from 2.52 Angstrom for the known Fe-2(CO)(9) to 2.00 Angstrom for the unbridged quadr uple bond species Fe-2(CO)(6). The lowest energy structure of Fe-2(CO)(7) i s a distorted unbridged C-s symmetry structure with iron-iron separation 2. 23 Angstrom. This is followed energetically by the tribridged structure wit h bond distance 2.21 Angstrom, and finally by the monobridged structure wit h iron-iron distance 2.13 Angstrom. The latter structure is consistent with an Fe=Fe triple bond, but is not a genuine minimum. For Fe-2(CO)(6) the lo west energy structure is the distorted dibridged structure with perhaps a w eak Fe=Fe double bond. However, the unbridged Fe-2(CO)(6) structure with an iron-iron bond distance of 2.00 Angstrom (suggesting a quadruple bond) is also a genuine minimum, The unsaturated structures Fe-2(CO)(8) and Fe-2(CO) (7), are thermodynamically resistant to CO removal. The iron-iron linkages are also analyzed in terms of contributions from the different vibrational potential energy distributions. A clear Badger's Rule correlation between F e-Fe vibrational frequency and bond distance is established. Prospects for the synthesis of these and related diiron compounds are discussed in some d etail. The most promising routes to preparation of these fascinating specie s would appear to be matrix isolation or iron vapor synthesis.