The homoleptic binuclear compound Fe 2 (CO) 9 is well characterized experimentally, although there has been some discussion as to the nature of the iron-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 formal double Fe 2 (CO) 8 (seven distinct structures), triple Fe 2 (CO) 7 (three distinct structures), and even quadruple Fe 2 (CO) 6 (seven distinct structures) iron-iron bonds. These novel structures are characterized in terms of their equilibrium geometries, thermochemistry, and vibrational frequencies. The range of predicted iron-iron distances is remarkable, from 2.52 Å for the known Fe 2 (CO) 9 to 2.00 Å for the unbridged quadruple bond species Fe 2 -(CO) 6 . The lowest energy structure of Fe 2 (CO) 7 is a distorted unbridged C s symmetry structure with ironiron separation 2.23 Å. This is followed energetically by the tribridged structure with bond distance 2.21 Å, and finally by the monobridged structure with iron-iron distance 2.13 Å. The latter structure is consistent with an FetFe triple bond, but is not a genuine minimum. For Fe 2 (CO) 6 the lowest energy structure is the distorted dibridged structure with perhaps a weak FedFe double bond. However, the unbridged Fe 2 (CO) 6 structure with an iron-iron bond distance of 2.00 Å (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 ironiron linkages are also analyzed in terms of contributions from the different vibrational potential energy distributions. A clear Badger's Rule correlation between Fe-Fe vibrational frequency and bond distance is established. Prospects for the synthesis of these and related diiron compounds are discussed in some detail. The most promising routes to preparation of these fascinating species would appear to be matrix isolation or iron vapor synthesis.
Ab initio calculations give, with an accuracy depending on the sophistication of the method, a bond length as an equilibrium value, r e. The experimental bond lengths are always vibrationally averaged and may be expressed in different ways (r g, r z, r a, etc.). Since high-quality ab initio calculations now are capable of giving bond lengths that are approximately of experimental accuracy, it is important to be able to interconvert these values. We find that the bond lengths optimized at the TZ2P+f CCSD level may be considered as the converged r e values and that the MM3 and MM4 force fields successfully convert r g to r e values. We also evaluated the performance of quantum mechanics at the 6-31G* MP2 and the 6-31G* B3LYP levels and found that the bond lengths (r e) at the 6-31G* B3LYP level are better than these at the 6-31G* MP2 level for molecules with only first-row atoms. However, the bond lengths for the bonds involving second-row atoms are too long at the 6-31G* B3LYP level, and for these, the 6-31G* MP2 level is recommended. An empirical formula is given for the conversion of the theoretical r e values calculated at these levels to the r g values.
A theoretical evaluation of tetra-tert-butylethylene (1) at the BLYP/DZd level confirms that it should be a stable molecule with a singlet ground state. The synthesis of 1 from two molecules of di-tert-butylcarbene (6) is unlikely. Although the formation of singlet 1 from the triplet 3B ground state of 6 (singlet 6 is only 1−3 kcal/mol higher in energy) is highly exothermic (ΔH = −73.7 kcal/mol), the barrier ΔG ⧧ = 25 kcal/mol (298 K, 1 atm, BLYP/DZd) for the dimerization is too large to compete with the barrier for intramolecular carbene insertion. The barrier for singlet 6 to yield 1,1-dimethyl-2-tert-butylcyclopropane (12) is only 5 kcal/mol. The CC double bond in singlet 1 is twisted by 45°, and the strain energy is ∼93 kcal/mol in agreement with molecular mechanics results. Triplet 1 has a nearly perfectly perpendicular conformation at the central CC bond (87° torsional angle), but it is still strained by 42 kcal/mol and is 12 kcal/mol higher in energy than singlet 1. Alkyl substitution decreases the S−T separation of carbenes due to the greater hyperconjugative stabilization of the singlet than the triplet.
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