Skip to main content



Carbonyl trapped in silico

When ligands bind to a transition metal its valence d shell gets split, which has interesting consequences if the shell is partially filled. Here, the energy differences between frontier orbitals can be small enough to make the metal a good σ-acid and π-base — useful properties when it comes to binding substrates such as H2 and CO. Chemists seek to replicate this behaviour using low-valent compounds of main-group elements, yet these species rarely have the appropriate atomic or electronic structure to afford stable adducts. A team led by Stephan Schulz and Peter Schreiner have hit back by preparing a silylene that enjoys steric and electronic protection in the form of bulky Ga-donor ligands. This privileged silylene, which the team now describes in Nature Chemistry, is ideally set up to split H2 or form a rare complex with CO.

Carbenes are likened to transition metals because C has two valence electrons to distribute between an sp2 hybrid orbital and a higher-lying p orbital. A common strategy to stabilize a carbene is to introduce π-donor substituents that increase the singlet–triplet energy gap, but this often robs the carbene of its ability to activate certain bonds. Enter the silylenes — divalent Si analogues of carbenes that exist as singlet species yet have distinct reactivity.

Credit: David Schilter/Springer Nature Limited

Early matrix-isolation work on the simplest silylene, H2Si:, uncovered a non-planar H2Si:–CO complex in which Si plays the synergistic role of σ-acid and π-base reminiscent of transition metals. Schulz, Schreiner and their colleagues used density functional theory calculations to show that this complex is 16 kcal mol−1 lower in energy than the silaketene H2Si=CO. By contrast, H2C: has greater access to its triplet state, such that its singly occupied sp2 and p orbitals engage in typical σ and π covalent bonding with CO to give ketene H2C=CO, which now lies 6 kcal mol−1 lower than the non-planar isomer H2C:–CO.

Despite its fragility, the observation of H2Si:–CO inspired the team to prepare a more stable analogue so they could probe its bonding and chemistry. For this they turned to other main-group chemistry they had developed, namely insertion reactions of unsaturated Ga species LGa (L = N,Nʹ-bis (2,6-diisopropylphenyl)-1,3-pentanediiminate). Thus, they converted SiBr4 to [L(Br)Ga]2SiBr2, a tetravalent species that undergoes debromination when treated with a further equivalent of LGa to generate a putative [L(Br)Ga]2Si: intermediate. If left to its own devices, the Si centre inserts into a C–C bond of its own ligand, so Schulz, Schreiner and colleagues conducted debromination under CO in hopes of trapping it as a complex (when under H2 the reaction affords silane [L(Br)Ga]2SiH2). Gratifyingly, the team isolated [L(Br)Ga]2Si:–CO and its X-ray structure reveals a non-planar arrangement that enables pSispC σ-bonding and synergistic sp2Si→π*CO backbonding. But the same arrangement exists in H2Si:–CO, so why is [L(Br)Ga]2Si:–CO stable at room temperature? Calculations on [L(Br)Ga]2Si: indicate that the bulkiness of the L(Br)Ga groups brings them apart and raises the energy of the sp2Si lone pair, which is now a better π-donor. Conversely, overlap of vacant p orbitals on the Si and Ga centres ensures that the pSi orbital is a good σ-acceptor. Curiously, π-bonding dominates, so it is arguable if we can even call it ‘backbonding’. “It took us a while to get our head around this but it is what makes bonding possible in the first place,” notes Schreiner.


Like many a metal carbonyl, [L(Br)Ga]2Si:–CO undergoes substitution with isonitrile CNCy to give [L(Br)Ga]2Si:–CNCy. “I am pretty sure that the mechanism is associative because it proceeds at 25 °C while the precursor is stable to decarbonylation up to 80 °C,” predicts Schulz. The team is now investigating ligand exchange and Si-assisted reactions at the CO ligand itself. This chemistry is a hallmark of transition metals and will no doubt motivate more investigations into such fertile main-group systems.


Original article

  1. Ganesamoorthy, C. et al. A silicon–carbonyl complex stable at room temperature. Nat. Chem. (2020)

    Article  PubMed  Google Scholar 

Download references

Author information



Corresponding author

Correspondence to David Schilter.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Schilter, D. Carbonyl trapped in silico. Nat Rev Chem 4, 274 (2020).

Download citation


Quick links

Nature Briefing

