N-Heterocyclic Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

Mirjam J. Krahfuß, Jörn Nitsch, F. Matthias Bickelhaupt, Todd B. Marder, Udo Radius*

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review


A study on the reactivity of the N-heterocyclic silylene Dipp2NHSi (1,3-bis(diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-yliden) with the transition metal complexes [Ni(CO)4], [M(CO)6] (M=Cr, Mo, W), [Mn(CO)5(Br)] and [(η5-C5H5)Fe(CO)2(I)] is reported. We demonstrate that N-heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi-bridged complex [{Ni(CO)2(μ-Dipp2NHSi)}2] (2), complexes [M(CO)5(Dipp2NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)3(Dipp2NHSi)2(Br)] (9) and [(η5-C5H5)Fe(CO)2(Dipp2NHSi-I)] (10). DFT calculations on several model systems [Ni(L)], [Ni(CO)3(L)], and [W(CO)5(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ- and π-interaction depends significantly on the system under investigation. The σ-interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π-interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ-donor ligand, and contributions of π-symmetry play only a minor role for the NHC and NHSi co-ligands.

Original languageEnglish
Pages (from-to)11276-11292
Number of pages17
JournalChemistry - A European Journal
Issue number49
Early online date31 Mar 2020
Publication statusPublished - 1 Sept 2020


This work was supported by the Julius-Maximilians-Universit?t W?rzburg and the Deutsche Forschungsgemeinschaft (DFG). Financial support from the Netherlands Organisation for Scientific Research (NWO) is gratefully acknowledged.

FundersFunder number
Deutsche Forschungsgemeinschaft
Nederlandse Organisatie voor Wetenschappelijk Onderzoek
Julius-Maximilians-Universität Würzburg


    • N-heterocyclic carbenes
    • silylene complexes
    • silylenes
    • stereoelectronic parameters
    • transition metal complexes


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