Formation of a four-electron donor carbonyl group in the decarbonylation of the unsaturated H 2C 2Fe 2(CO) 6 tetrahedrane as an alternative to an iron–iron triple bond

Abstract

The unsaturated Fe 2C 2 tetrahedrane derivatives R 2C 2Fe 2(CO) 6 (R = Ph, t Bu) are among the many products obtained from reactions of the alkynes RC CR with iron carbonyls. In this connection theoretical studies have been performed on the simplest such compounds H 2C 2Fe 2(CO) n ( n = 6, 5) for comparison with the experimentally known structure of the t-butyl derivative t-Bu 2C 2Fe 2(CO) 6 and in order to predict the decarbonylation pathways for such (alkyne)Fe 2(CO) 6 derivatives. These theoretical studies predict an Fe 2C 2 tetrahedrane structure for H 2C 2Fe 2(CO) 6 with a formal Fe Fe double bond very similar to the experimental structure for t-Bu 2C 2Fe 2(CO) 6. Decarbonylation of H 2C 2Fe 2(CO) 6 is predicted to give an H 2C 2Fe 2(CO) 5 isomer retaining the Fe 2C 2 tetrahedrane structure, with an Fe Fe double bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M 2C 2 tetrahedrane structure. The formation of formal Fe Fe triple bonds appears to be avoided in even the higher energy H 2C 2Fe 2(CO) 5 structures. These include three triplet Fe 2C 2 tetrahedrane structures with formal Fe Fe double bonds as well as a coordinately unsaturated singlet structure, still with an Fe Fe double bond.