Inorganometallic allenes [(Mn(η 5 -C 5 H 5 )(CO) 2 ) 2 (μ-E)] (E = Si-Pb): bis-allylic anionic delocalisation similar to organometallic allene but differential σ-donation and π-backdonation.

The chemistry of heavy group-14 tetrel atoms is known to diverge from that of the lighter congener carbon. Here, we report the structure and bonding in inorganometallic allenes [(MnCp(CO) 2 ) 2 (μ-E)] (2E, E = Si-Pb; Cp = η 5 -C 5 H 5 ). These inorganometallic allenes are structurally similar to the lighter organometallic analog [(MnCp(CO) 2 ) 2 (μ-C)] (2C). The bonding analysis of these compounds at the M06/def2-TZVPP//BP86/def2-SVP level of theory identifies a linear Mn-E-Mn spine with delocalised, mutually orthogonal π-systems across this back-bone. This results in a bis-allylic anionic bonding scenario. However, the strength of the Mn-E bonding is found to be weaker in these inorganometallic allenes. The energy decomposition analysis at the BP86/TZ2P//BP86/def2-SVP level of theory further reveals that the bonding in these compounds cannot be represented by one unique heuristic bonding model, but multiple bonding models. For all 2E (E = C-Pb), the Dewar-Chatt-Duncanson bonding model is one of the best bonding representations, where the central tetrel atom acts as a 4e - σ-donor and 4e - π-acceptor. The bonding analysis indicates that the carbon atom in the organometallic allene acts as a better π-acceptor than σ-donor, while the heavier tetrel atoms in the inorganometallic allenes are better σ-donors than π-acceptors. The n p z -orbital is found to be a better σ-donor than the valence n s-orbital. However, when the bonding representation is changed to a traditional electron-sharing model, the contribution from the n s-orbital was found to be the largest in comparison to the interaction from the remaining three valence n p-orbitals. It can be suggested that the n s-orbitals contribute more towards chemical bonding when participating via an electron-sharing interaction than a donor-acceptor interaction.