Inorganometallic allenes [(Mn(η5-C5H5)(CO)2)2(μ-E)] (E = Si–Pb): bis-allylic anionic delocalisation similar to organometallic allene but differential σ-donation and π-backdonation

Abstract

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 = η⁵-C₅H₅). 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 np_z-orbital is found to be a better σ-donor than the valence ns-orbital. However, when the bonding representation is changed to a traditional electron-sharing model, the contribution from the ns-orbital was found to be the largest in comparison to the interaction from the remaining three valence np-orbitals. It can be suggested that the ns-orbitals contribute more towards chemical bonding when participating via an electron-sharing interaction than a donor–acceptor interaction.