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 RCtriple bond; length of mdashCR 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 Fedouble bond; length as m-dashFe 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 Fedouble bond; length as m-dashFe double bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M2C2 tetrahedrane structure. The formation of formal Fetriple bond; length of mdashFe triple bonds appears to be avoided in even the higher energy H2C2Fe2(CO)5 structures. These include three triplet Fe2C2 tetrahedrane structures with formal Fedouble bond; length as m-dashFe double bonds as well as a coordinately unsaturated singlet structure, still with an Fedouble bond; length as m-dashFe double bond.