Formation of a four-electron donor carbonyl group in the decarbonylation of the unsaturated H2C2Fe2(CO)6 tetrahedrane as an alternative to an iron-iron triple bond

The unsaturated Fe₂C₂ tetrahedrane derivatives R₂C₂Fe₂(CO)₆ (R = Ph, ^tBu) are among the many products obtained from reactions of the alkynes RCCR with iron carbonyls. In this connection theoretical studies have been performed on the simplest such compounds H₂C₂Fe₂(CO)_n (n = 6, 5) for comparison with the experimentally known structure of the t-butyl derivative t-Bu₂C₂Fe₂(CO)₆ and in order to predict the decarbonylation pathways for such (alkyne)Fe₂(CO)₆ derivatives. These theoretical studies predict an Fe₂C₂ tetrahedrane structure for H₂C₂Fe₂(CO)₆ with a formal FeFe double bond very similar to the experimental structure for t-Bu₂C₂Fe₂(CO)₆. Decarbonylation of H₂C₂Fe₂(CO)₆ is predicted to give an H₂C₂Fe₂(CO)₅ isomer retaining the Fe₂C₂ tetrahedrane structure, with an FeFe double bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M₂C₂ tetrahedrane structure. The formation of formal FeFe triple bonds appears to be avoided in even the higher energy H₂C₂Fe₂(CO)₅ structures. These include three triplet Fe₂C₂ tetrahedrane structures with formal FeFe double bonds as well as a coordinately unsaturated singlet structure, still with an FeFe double bond.