Revisiting CNC₆F₅: The Quest for Isocyanide Ligands with Strong π‑Acceptor Properties Evaluated by Energy...

While perfluorinated isocyanide ligands such as CNCF₃ and CNC₆F₅ have been known for decades, their use by organometallic chemists has been limited primarily due to the challenges associated with their cumbersome synthesis. In this study, we present an improved synthetic route to [Cr-(CO)₅(CNC₆F₅)] and present its structural characterization. For a set of isocyanide ligands (CNC₆H₅, p-CNC₆H₄F, CNCH₃) and their perfluorinated counterparts (CNC₆F₅, CNCF₃), Gibbs energies of complexation have been calculated with regard to a series of isoelectronic metal fragments [V-(CO)₅]^-, [Cr-(CO)₅], [Mn-(CO)₅]⁺, and [Fe-(CO)₅]²⁺. Furthermore, the σ-donor and π-acceptor properties of these isocyanide ligands in the resulting complexes were analyzed using the EDA-NOCV method. For completeness, we have also included ligands such as CO, CNH, and N₂ into the analysis. While only minor differences in complexation energies are observed for the Cr-(CO)₅ fragment, more pronounced effects have been observed for the charged complexes. Interestingly, perfluorinated isocyanide ligands show in all cases higher complexation energies than the carbonyl ligands, indicating their strong binding to metal centers. Their pronounced σ-donor and π-acceptor abilities reveal their potential suitability to stabilize metal centers in both positive and negative oxidation states.