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.

Density functional theory predicts 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 a four-electron donor bridging carbonyl group.Figure optionsDownload as PowerPoint slide