Diversity in organometallic allenes: A structure-bonding study on two isomeric organometallic allenes [(μ-C)(Fe(CO)₄)₂] reveals a bis-pseudoallylic anionic delocalisation, similar to organic allene C(CH₂)₂, in the first case, and a typical three-center bis-allylic anionic delocalisation in the second one. A quantitative bonding analysis shows the transformation of the central carbon atom from a classical tetravalent coordinating center to a double σ-donor double π-acceptor ligand.

Abstract

Allenes (R₂C=C=CR₂) have been traditionally perceived to feature localized orthogonal π-bonds between the carbon centres. We have carried out quantum-mechanical studies of the organometallic allenes envisioned by the isolobal replacement of the terminal CH₂ groups by the d⁸ Fe(CO)₄ fragment. Our studies have identified two organometallic allenes viz. D_2d symmetric [(μ-C)(Fe(CO)₄)₂] (2) and D₃ symmetric [(μ-C)(Fe(CO)₄)₂] (3) with trigonal bipyramidal coordination at the Fe atoms. Compound 2 features the bridging carbon atom in an equatorial position with respect to the ligands on the TM centre, while 3 features the central carbon atom in an axial position. The bis-pseudoallylic anionic delocalisation proposed in the C2-C1-C3 spine of organic allene is retained in the organometallic allene 2, and is transformed to a typical three-centre bis-allylic anionic delocalisation in the organometallic allene 3. The topological analysis of electron density also indicates a bis-allylic anionic type delocalisation in the organometallic allenes. The quantitative bonding analysis using the EDA-NOCV method suggests a transition from classical electron-sharing bonding between the central carbon atom and the terminal groups in 1 to donor-acceptor bonding in 3. Meanwhile, both electron-sharing and donor-acceptor bonding models are found to be probable heuristic bonding representations in the organometallic allene 2.